EBAY: SBC 486 50MHz (not mine)

[Zac67]

You can also get Pentium boards in that form factor, some of them with Ethernet NIC...

[Zac67]

You could probably make a bridgeboard that worked alongside one of these cards. But finding someone that can do it and will put the effort in is the problem. There is software for linking PC's and Amiga's though, which should work.

Most probably it's so much easier to link them via Ethernet without any special hardware (and probably not much slower either). A bridgeboard requires dual-ported RAM and lots of protocols and system software on both sides.