The problem is this: the rear wheel is rolling. As we all know, a traditional freewheel, when the wheel rolls backwards, will engage the pawls and rotate the cranks backwards.

If you pedal the cranks backwards faster than the freewheel takes up the chain, then you continue to freewheel.

However, if you use mismatched freewheels for different gear ratios, one of them will effectively cause the other to be rotating backwards faster than the chain allows and will bind up the drivetrain as it tries to pull the cranks along with it.