This is a question I have struggled with over the years, and I still am not sure I know a definitive answer. One of my favorite fugues is the double fugue in F (BWV 540), which presents similar difficulties. I've studied with teachers (including one famous one) who insisted that the tempos should be the same for both fugues. On the other hand, they also insisted when playing Rheinberger that the tempo should remain the same throughout a single movement. I could never get that to work, and finally realized through research that it is very likely that Rheinberger WOULD have varied tempos in a single movement. I know of no evidence proving that Bach would NOT have varied tempos, but I know of nothing proving that he DID, either. When I play BWV 540, I still go back and forth on how I do it, but the way I do it most often is with a slightly faster tempo for the second fugue. I have not performed BWV 552, so I can't say for sure, but I might consider a slight change of tempo. BUT I don't think it should be drastic. I think it should be a subtle change that the audience probably will not notice. If it's too noticeable, or a real "bump" I think it's probably too much. On the other hand, I would still experiment with trying to find the one consistent tempo that works. I still do that sometimes with BWV 540. It's possible that Bach did NOT want a tempo change, and that it's MY concept of how the sections should go that is wrong. We need to always consider this. But I cannot say that, at this point, I know which is "right."