The BBC micro family (6502 processor) had a [tube interface][1] which permitted Z80, 6502, or [80186 coprocessors][2] (possibly others). See e.g. [here][3].

Each of these consists of a 'cheese wedge' with a board in with its own CPU and RAM. Both processors thus can run simultaneously (though that wasn't the normal intention which is why some of the links describe the coprocessor as *'running the user's program'*), and indeed do when e.g. accessing disks etc.


 [1]: https://en.wikipedia.org/wiki/Tube_(BBC_Micro)
 [2]: http://www.cowsarenotpurple.co.uk/bbccomputer/master512/tube.html
 [3]: http://www.beebmaster.co.uk/SecondProcessors.html