Doing the math, you are already at 50 teeth of derailleur capacity. That's a lot. Off hand the most capable derailleur I can think of is Shimano XTR M-9000-SGS Shadow Plus. & it's capacity is 47. I'm sure that capacity can be found in lower tier models by now.
My point is findind a lower climbing gear is going to require a smaller crankset all the way around. Meaning a smaller big ring too. You will be trading the high end for more low.
Irrespective of mechanical considerations, at some point it becomes very difficult to get enough purchase with each pedal stroke to stay upright. On a lone mountain bike, my own personal limit is about 15 gear inches. At that point either the front wheel comes off the ground, the rear is limited by traction, or the bike is so slow balance becomes an issue...& you can rule out starting out in a gear that low. I can't imagine a loaded tandem would be any easier.
I think you are at about the limit of practicality with out dumping serious cash on a derailleured Rohloff type situation.