Now that I have a year on a 29" full suspension bike, I'd not hesitate for a second to go with a 29" tandem. My only reason for going 27.5" would be if proper fit couldn't be achieved with 29". My stoker is 5', so it made total sense for her to go with 27.5" wheels on her new single full suspension bike. But if you CAN ride 29" wheels, why not? My next mountain tandem will be a 29" for sure, probably a Ventana ECDM.
And as mentioned earlier, you don't have the luxury of lifting the whole bike up and over many obstacles, so the "rollover" advantages of the larger wheels is a huge plus. Those large wheels plus 160mm suspension travel? Unstoppable! That said, I lift the tandem front wheel off the ground all the time, so I don't get the "you can't lift the front end" comment. We bunnyhop too, but it's virtually useless as you can't clear anything with the long wheelbase.
And if doing marathon races, then this further justifies the larger wheels.
Slow steering? Sorry, I don't really see it. Tight, technical single track has been zero issue on the 29" full suspension for me so far. Tahoe Rim Trail, Nevada City's Harmony Ridge (Hoot, Scotts Flat, and now Talon Show) are all the best! It's way, way better with the larger wheels. I feel the heavier weight of the larger wheels, heavier tires and sealant, for sure. But that's a trade-off I'm more than happy to accept. (****, I took an 11 lb. weight penalty going from my ti hard tail to the Ibis. Totally worth it!) I rode Hole in the Ground at Donner Pass last October. (I've done it several times on the 26" single and 26" tandem.) But it had been a few years since I had ridden it; my fitness just wasn't there. So I head on up with much better fitness to ride it again - and on the new Ibis. How exciting! Was really looking forward to it. But at 56 yo, I took it with a cautious approach: I won't pressure myself to try to ride everything. Get off and walk to be able to ride another day type of thinking. (I enjoy mellow observed trials goofing around; I like technical challenges.) Despite riding with a partner, I'd still take it easy. Well, I quickly found the 29" wheel so sweet, I attempted everything. Cleaned it all, save for three spots where my fitness and the altitude gassed me. (My friend on his 26" bike went OTB, BTW! Glad he was basically uninjured.)
So with my experience on the 29" wheels and no size constraints (5' 9" riding size M or L), it would be a 29" tandem without much more thought.