Whilst MOS Technology's 6502 and Zilog's Z80 dominated the early microcomputer and home markets, the two were generally considered to be on opposite sides of the fence, or even arch enemies.

The 6502 was particularly popular in home computers, whilst the Z80 - although appearing in Sinclair's popular ZX machines - was often considered to be more "businessey", thanks to its symbiotic relationship with CP/M.

As such, there were a few "multi-processor" machines around - not in the modern sense with a multi-core CPU, but literally with two or more completely different processors, used together to get the best of both worlds. 

One such combination was the 6502, which thanks to the way it was designed could be made to work very fast with video memory, along with the Z80 to give access to a huge library of CP/M business software.