Combining Visiting Professor with TT
I will answer this question in light of @paulgarrett 's comments:


A Visiting Professor position is a full time job and not combineable with a full time TT position
A Visiting Professor position is no guarantee whatsover for anything else in the future so one should not give up her TT position for this.
Visiting Scholar seems to be the type of position that is combineable: you start a research collaboration with the second university but officially affiliate 100% with your current one.


I ended up going for something in between which I hope will combines the best of both worlds: having the visiting scholar position for a year and simultaneously giving up 20% of my TT at my current institution in order to get a temporary 20% position as visiting professor at the external institution.