I wonder about absorbing the chain slack on a Birdy with a double ring in the front. The somewhat unusual tensioner on Brompton absorbs plenty of slack and I was able to get away with a triple ring in the front without essentially any modifications regarding the slack - I can actually fold the bike with the chain on the smallest ring in the front. However, when I replaced the rear Brompton derailleur with a regular one, like on a Birdy, I ran into problems with the slack for a folded bike. I made a chain catcher, such as one I saw in photos of some Birdys, but it only helped with one ring in the front. Two or more rings were turning into disasters. You could always fold, of course, exclusively with the chain on the largest ring, but it is a hassle.