Can't share returns.

why?

One unit will suck air backwards through the other one.

Yes, but, if both returns are long duct runs and both meet at a Y at the grille, I don't see how that would be a problem. Maybe I wasn't clear with my initial question.

That take's care of free advice,hire a ventilation engineer to come out for a secound opinion. Anything is possible but it would require duct dampers controlled by one of the units. Granted the effect is progressivly less as junction moves closer to intake grill but still ill advised.

My two furnaces share a common return. This is how the house was built 32 years ago. When I replaced the furnaces the HVAC contractor installed two dampers each controlled by one furnace. I was told was needed in order to keep the blower motors from spinning backwards when only one furnace is on. The dampers are normally closed and open when the furnace starts up.

Squirrel cage blowers don't move much air in reverse rotation and very little. They move what little air they do in the same direction no matter what direction the rotation is. Corollary to that, there will not be much resistance to air flow in the reverse direction so there will be little force generated to spin the motor. The cage and motor will be turning in the same direction if it turns at all.

Depending on how the duct system is designed, as already pointed out, you have the potential to turn supplies into returns.