After some more discussion with my friend David, we decided last night that it wouldn't take a whole lot more effort to build a prototype that supports both 2600/7800 ports as well as both the 2-port and 4-port version of the Atari 5200.

In addition to my 7800, I have a 4 x port 5200, so I can test the full-range of the prototype.

This can be done with 4 x 5242 chips, 13 x 715 chips, 1 x multiplexer, and power switch.

I ordered all the necessary parts today, several additional chips and two more 5200 controller cables.