I think the other part of hill climbing has to do with unequal power. One of you is likely to need a higher cadence than the other to maintain power climbing a hill without damaging knees. The one with the lower desired cadence can either insist on the lower cadence and do ALL the work (since the other has to lay off at that point) or accept the higher cadence and push with less than full power (for that rider). Either way, it is hard to find the perfect balance when climbing. So you can get into a situation where one is close to optimum ability to provide power but not the other. The weight of both riders and the bike, but less than the power of both riders. So it can be harder to get up the hill than it would be on two singles.

Happily all of that only applies to up hill riding. Down hill and level, even up wind, the extra power tends to be more than in proportion to the extra weight, since the wind resistance doesn't go up much.

All of that said, in the original question I think you appeared to be contemplating getting closer gearing. With the available range of a DaVinci, you should be able to afford to lose some of the bottom end, and close gearing is especially nice on a tandem. For reasons I've not fully come to understand, our experience - widely corroborated - is that you shift more frequently on a tandem. We aren't as easily satisfied with the gear choice as we would be on a single. I use each and every cog on the tandem, making frequent single gear shifts. On a single, I make far fewer single gear shifts. Perhaps I'm just not paying as much attention.