For us, we kept the original stoker crank, FSA Gossamer MegaEXO tandem triple/bottom bracket. Bike is circa 2014 and this crankset appears to still be available. Original gearing was 28-39-52 and new is 34-50. Bolt pattern is 74 for the granny and 130 for the center and outer. You shouldn't need to worry about chainring spacing when going from 10 to 11 speed. I don't know if this holds true for going from 9-11 or others, a google search would be in order for your case. Adjustment of the FD was a breeze, and especially so that when you have to move its position on the seat tube, you don't have a taught cable to deal with. For chainrings we have a 50T pinned and ramped ring from Origin8 and the 34 is a custom made ring from House of Tandems. For our bike HOT recommended utilizing the inner and center ring positions. As for the position of the third chainring, all we had to do was remove it and use shorter chainring bolts.
The bigger question you are asking, going from 3x to 2x up front, is the one I struggled with for a few years until deciding to make the plunge. This is a question that leaves many tandem riders concerned, especially when thinking about dropping the kind of money as being contemplated here. In summary, I like the new set up much better in practice than I believed it was going to be. In fact, with the large rear cassette, many smaller hills that we would downshift to the middle ring we no longer have to and just stay in the larger ring. About a year or so ago, this transition was discussed on this forum and I did a spreadsheet analysis as to how large a change between adjacent gears actually was. By adjacent gears, I assumed that in all shifting, either in front or in rear, would occur one derailleur at a time. For example, say you were going from 50 to 34 up front, and 28 to 24 in the rear. In almost all cases, one would shift one of the derailleurs first, then shift the other, one would rarely shift both at exactly the same time. With this assumption, I learned that no shifts were greater than 6 teeth in difference in both drivetrains. What we have experienced is that the triple gives you more granularity (and overlap) than the double, however, the action of a shift is no more impactful in either set up. On top of that, the electric shift is so effortless and crisp, it becomes just a push of a button over a contemplation of a shift, waiting for the cranks to be in the optimal position, executing the shift. You just push the button and it goes there. I think the key to all of this is to utilize a wide range cassette with a road-link derailleur hanger. You can easily create a spreadsheet comparing gearing in both current and planned set ups and I think you will find as we did that we gave a little on the high end, picked up a little on the low end which we wanted, and overall it was a better experience. Do pay attention to system capacity.
I don't know if I articulated this clearly, perhaps others can add their words and a good picture can be drawn, however, bottom line, it was not as impactful as I anticipated, and is actually better than I thought it would be.