The Rockwell R65C00/21 and R65C29 are described in an October 1984 datasheet ("R65C00/21 dual CMOS microcomputer and R65C29 dual CMOS microprocessor," Document no. 29651N64, rev. 2) which is Section 3-3 of the Rockwell 1985 Data Book.
The R65C00/21 "consists of two enhanced instruction set 6502 CPU's in one device" (the additional instructions include a 10-clock time multiply). The two CPU's are "functionally independent" with each having "its own set of registers, its own reset and interrupt vectors and operates under control of its own program" but with both sharing "the same memory and system I/O resources." The R65C29 is a ROM-less version of the R65C00/21.
(Note that some sources, such as Wikipedia, refer to the R65C00/21 as two chips, the R65C00 and the R65C21, but I believe that this is an error, that there is only one chip and that its name includes the slash. The datasheet I cited never talks about an "R65C00" or "R65C21.")
Is there any system (commercial or otherwise) that used the R65C00/21 or the R65C29?