Are you any good with electronics & a soldering iron?
If I understand what you want to do, you want ONE footswitch to control TWO amps.
One way to do this would be to build a footswitch with two cable connectors, one to each amp. Conceptually, imagine replacing the footswitches in the TWO footswitch floor units with relays. Then buld a third footswitch unit with buttons to toggle the relays in tandem, and wire everything up. Now imagine putting all of this in one floor box.
Custom Audio Electronics could do this for you, but it would probably cost quite a bit.
You might be better off just putting the two footswitch boxes close enough together that you can hit both at the same time with the same foot.
Another way is to use one power amp section as a slave to the other amp. In other words, you use the preamp and tone control of one amp, but the power amps and speakers in both. If you have a rack effects unit, you might also be able to make this rig true stereo as opposed to dual mono.