If you are like most teams your tandem climbing style does not involve much out of the saddle honking in the cruising ring, and much more of a 'sit and spin" (if it can be called spinning) in the granny ring. On your road bike you attack the climb with plenty of momentum to start and stand up as it bleeds off. If the hill is short enough you may never have to shift down. That is why a 34Tx28T low gear combination can serve well on a roadbike. We have had a number of tandems and all of them, including the double recumbent tandem we just started riding, all of them, first thing we do is change the granny to 24T (they are usually 30T as sold). You will regret not going all the way down to 24T more than you might have regretted going to 28T instead of 26T.. 24T is the lowest possible granny with a 74mm bolt circle spider which yours almost certainly is. If you are going through the hassle of pulling the crank and swapping out the rings you may as well go all the way down. We have both a 52/42/24 and a 52/39/24 on different tandems. The shifting from the granny to the middle ring is equally easy for both, but we prefer the feel of the shift to the 39T cruising ring and I am looking around for another so both tandems will have 39T cruising rings. FWIW.