Greg is quite right, HB. There is no easy way to do what you want without some seriouly warranty voiding modifications to some of the equipment, like rewiring the cabs to lesser impedance or replacing some, or all, the speakers.
However, you can very easily
connect two stock 210 cabs to the
Rumble Stage 800 bass modeling combo (same dimensions as the R500c), since it can handle a 4 ohm total extension cab load by switching the Impedance Switch:
In fact, I've been encouraging our Rumble Stage owners to post a pic of such a triple decker stack! But so far, no takers. Maybe you could be the one! Just be forewarned that the bass
modeling Stage (and the smaller Studio 40) are totally different beasts from all the other Rumbles!!!
.