The question is a bit unclear(*1) about the margins set regarding:
- Must it be a single motherboard or do separate assemblies qualify?
- Must it be PC-compatible or does any x86 system qualify?
- Must the board have been available separately (to the general public) or do complete assembled systems qualify?
So answers do vary a lot depending on what criteria are to be applied, offering year tags from 1979 all the way to 1995. Pick your choice.
Three Quick Answers:
- If it's just about being x86 and SMP, then the earliest 8086 multibus boards will qualify - even way before the PC.
- If it's about PC compatibility, then the dual 80386 configured Compaq SystemPro will make it.
- In the most strict sense of the above criteria, then the motherboards found by Stephen Kitt are the answer you're looking for.
The Long Read
The original 8086 was already not only capable of sharing the bus between multiple CPUs, it was a core feature to enable operation of configurations with an 8089 I/O processor - as well as multiple CPUs and multiple I/O processors in one system. It was there from the very beginning. In fact, the first ever systems made by Intel for Multibus have been used in SMP configurations.
Intel can't really be blamed for IBM using the CPU in a somewhat crippling design.
Multiprocessor support on the IBM PC-based platform, as far as I know, required BIOS support and additional hardware support, and introduced things like the MPS specification and the APIC.
Not really. It just simplifies standard usage.
So which motherboard was the first to support multiple CPUs and SMP?
Well, ignoring pre/non-PC MP systems like Sequent's 80386-based Symmetry systems (1987), the dual 80386 Compaq SystemPro of 1989 might be a good starting point for dual CPU and PC compatible. That was before the APIC and way before MPS. Though, I'm not really sure if it does fully fit the SMP category, as the second CPU acted as a dedicated I/O processor running driver code; it didn't take any user tasks. So while memory access was common to both, processing was asymmetric.
Then again, according to a quite interesting benchmarking project (found by Stehen Kitt), Windows NT 3.1 does seem to recognize both CPUs, so there might be more research necessary.
The very same machine could be ordered after 1990 with dual 80486 which also introduced APIC (*2). But APIC was restricted to Intel systems as its workings were patented - AMD and other compatible manufacturers introduced openPIC, but with limited success (*3). There was simply no real need for SMP when it came to machines that had to be 100% compatible, while at the same time multi-CPU servers were a special case anyway. This continued with the Pentium (*4). Fully PC-compatible systems stayed rather rare.
The restriction to a 'motherboard' makes it a bit problematic, as the SystemPro has the CPUs plus all arbitration logic on a sub-assembly. The motherboard itself is agnostic of the number of CPUs installed. So again, not sure if it counts.
It only took off with the Pentium Pro, after 1995, when Intel pushed dual socket systems for high end PCs. Almost every major manufacturer had one or more such system made close to Intel's specifications, including the separate voltage converter modules(*5). Compaq's 273708-001 board may serve as a good example for a workstation - here both sockets and all logic are on the motherboard. On the server side there was a dual PPro card for the ProLiant 5500 Model 6/200 (type 285100-001), which could take two of them making it a quad CPU system, but that would again be not on the motherboard.
By the time of the Pentium II, AMD licensed APIC from Intel for their K7 design and the rest is history...
A notable mention should go here as well for Intel's iPSC of 1985. Although not SMP, the system is, with up to 128 80286 CPUs, the first major example for using off-the-shelf x86 components to build a supercomputer, something standard in today's high-end world. (These x86-based systems are not really PC compatible, even though they are standard x86 SoCs.)
*1 - It's not always easy to set a history question from today's POV, is it?
*2 - For the advent of multi-processor systems, Intel's MPS of 1994 was way more important than the APIC hardware itself. Before that, everyone tried their own methods for communication.
*3 - Interestingly, IBM, Apple and Motorola implemented openPIC in the form of MPIC for PowerPC based systems, like various RS/6000 or Macs.
*4 - At this point Alan Cox might be helpful, as he, AFAIR, brought MPS for the Pentium into Linux.
*5 - One of the hassles of finding PPro System is when they are taken, as it renders the machine useless. This is quite similar to 68k Macs, where the socketed system ROM was taken out by someone salvaging RAM modules.