My way of doing it is to use a DPM, one side to the bus, the other to the '51.
No timing to resolve, no synchronization problems, easy, reliable. Also you can debug either side without upsetting (save a possible stay in a loop) the other. Populate the board with the DPM and the bus logic only, make sure that does not kill the PC, verify you can read and write the DPM from the PC - one side debugged.

Erik