Thanks for your answer!
Yes, I know that the control system will be one of the challenges. I have seen here that some guys are playing with arduino control boards, I have thought maby that could be a route to go. I don't know very much about arduino though.
The thing is that i don't have very much yet. The outdoor unit is something I saw for sale, and the evaporators probably aren't usable since they are 42 years old, and most likely for R12 or R22. The fan motors are probably 115 volt, and I need 220 volt.
But I still wanted to make a tread about diy multi-split, to see if there are others with similar dreams, or just to gather info about this. I can't believe I'm the only one in the world wanting to try turning a single split into a multi-split of some kind. Maby someone allready has tried something similar, and can tell me and others about all the pits not to fall into.