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If you’re looking specifically for motherboards directly supporting multiple x86 CPUs, in a multiprocessor configuration, and available for purchase outside the system they were designed for, a likely candidate for the first such motherboard is the Gigabyte GA-586ID, which supported two socket 5 Pentium CPUs in an SMP configuration. It was released in 1994. Supermicro and Tyan apparently produced dual-486 motherboards too but I haven’t found them yet. There were quite a few dual-Pentium motherboards, see this post for a selection (thanks to snips-n-snails for the link).

There were multiprocessor x86 systems available for purchase earlier than that, as described in the other answers. They typically used per-CPU daughtercards. Multiprocessing on PC-based x86 only became possible in generic systems with the advent of the MultiProcessor Specification in 1994; before that, operating systems needed hardware-specific support (as was the case with Windows NT 3.1 on the SystemPro).

There were also motherboards which supported multiple CPUs, but only used one at a time; I used to have a Forex board which could use a 386 or 486. Arguably any 486SX motherboard with a socket for a 487 qualifies in this category, since the 487 is a full-blown 486DX, so a 486SX with a 487 is a system with two CPUs, only one of which is used.

If you’re looking specifically for motherboards directly supporting multiple x86 CPUs, in a multiprocessor configuration, and available for purchase outside the system they were designed for, a likely candidate for the first such motherboard is the Gigabyte GA-586ID, which supported two socket 5 Pentium CPUs in an SMP configuration (one P54C and one P54CM). It was released in 1994. Supermicro and Tyan apparently produced dual-486 motherboards too but I haven’t found them yet. There were quite a few dual-Pentium motherboards, see this post for a selection (thanks to snips-n-snails for the link).

There were multiprocessor x86 systems available for purchase earlier than that, as described in the other answers. They typically used per-CPU daughtercards. Multiprocessing on PC-based x86 only became possible in generic systems with the advent of the MultiProcessor Specification in 1994; before that, operating systems needed hardware-specific support (as was the case with Windows NT 3.1 on the SystemPro).

There were also motherboards which supported multiple CPUs, but only used one at a time; I used to have a Forex board which could use a 386 or 486. Arguably any 486SX motherboard with a socket for a 487 qualifies in this category, since the 487 is a full-blown 486DX, so a 486SX with a 487 is a system with two CPUs, only one of which is used.

If you’re looking specifically for motherboards directly supporting multiple x86 CPUs, in a multiprocessor configuration, and available for purchase outside the system they were designed for, a likely candidate for the first such motherboard is the Tyan S1462, which supported two socket 5 Pentium CPUs in an SMP configuration. It was released in 1995 (I think). Supermicro and Tyan apparently produced dual-486 motherboards too but I haven’t found them yet.

There were multiprocessor x86 systems available for purchase earlier than that, as described in the other answers. They typically used per-CPU daughtercards. Multiprocessing on PC-based x86 only became possible in generic systems with the advent of the MultiProcessor Specification in 1994; before that, operating systems needed hardware-specific support (as was the case with Windows NT 3.1 on the SystemPro).

There were also motherboards which supported multiple CPUs, but only used one at a time; I used to have a Forex board which could use a 386 or 486. Arguably any 486SX motherboard with a socket for a 487 qualifies in this category, since the 487 is a full-blown 486DX, so a 486SX with a 487 is a system with two CPUs, only one of which is used.

If you’re looking specifically for motherboards directly supporting multiple x86 CPUs, in a multiprocessor configuration, and available for purchase outside the system they were designed for, a likely candidate for the first such motherboard is the Gigabyte GA-586ID, which supported two socket 5 Pentium CPUs in an SMP configuration. It was released in 1994. Supermicro and Tyan apparently produced dual-486 motherboards too but I haven’t found them yet. There were quite a few dual-Pentium motherboards, see this post for a selection (thanks to snips-n-snails for the link).

There were multiprocessor x86 systems available for purchase earlier than that, as described in the other answers. They typically used per-CPU daughtercards. Multiprocessing on PC-based x86 only became possible in generic systems with the advent of the MultiProcessor Specification in 1994; before that, operating systems needed hardware-specific support (as was the case with Windows NT 3.1 on the SystemPro).

There were also motherboards which supported multiple CPUs, but only used one at a time; I used to have a Forex board which could use a 386 or 486. Arguably any 486SX motherboard with a socket for a 487 qualifies in this category, since the 487 is a full-blown 486DX, so a 486SX with a 487 is a system with two CPUs, only one of which is used.

If you’re looking specifically for motherboards directly supporting multiple x86 CPUs, in a multiprocessor configuration, and available for purchase outside the system they were designed for, a likely candidate for the first such motherboard is the Tyan S1462, which supported two socket 7 Pentium CPUs in an SMP configuration. It was released in 1995 (I think). Supermicro and Tyan apparently produced dual-486 motherboards too but I haven’t found them yet.

There were multiprocessor x86 systems available for purchase earlier than that, as described in the other answers. They typically used per-CPU daughtercards. Multiprocessing on PC-based x86 only became possible in generic systems with the advent of the MultiProcessor Specification in 1994; before that, operating systems needed hardware-specific support (as was the case with Windows NT 3.1 on the SystemPro).

There were also motherboards which supported multiple CPUs, but only used one at a time; I used to have a Forex board which could use a 386 or 486. Arguably any 486SX motherboard with a socket for a 487 qualifies in this category, since the 487 is a full-blown 486DX, so a 486SX with a 487 is a system with two CPUs, only one of which is used.

If you’re looking specifically for motherboards directly supporting multiple x86 CPUs, in a multiprocessor configuration, and available for purchase outside the system they were designed for, a likely candidate for the first such motherboard is the Tyan S1462, which supported two socket 5 Pentium CPUs in an SMP configuration. It was released in 1995 (I think). Supermicro and Tyan apparently produced dual-486 motherboards too but I haven’t found them yet.

There were multiprocessor x86 systems available for purchase earlier than that, as described in the other answers. They typically used per-CPU daughtercards. Multiprocessing on PC-based x86 only became possible in generic systems with the advent of the MultiProcessor Specification in 1994; before that, operating systems needed hardware-specific support (as was the case with Windows NT 3.1 on the SystemPro).

There were also motherboards which supported multiple CPUs, but only used one at a time; I used to have a Forex board which could use a 386 or 486. Arguably any 486SX motherboard with a socket for a 487 qualifies in this category, since the 487 is a full-blown 486DX, so a 486SX with a 487 is a system with two CPUs, only one of which is used.