I wonder if it is somehow possible to place the outgoing part of an underground belt first, because sometimes i build my belts backwards and it would be also useful to split the right and left side of a belts load by just placing a single (instead of both) part of the underground belt. Anyway i tried a lot of things like rotating, replacing etc. but i don't know how to place the outgoing part first.
No, you cannot place the outgoing belt first. However, you can actually build the belt backwards, then, without holding any items in your cursor, hover over either end of the underground belt and hit the “Rotate” key (R by default). This will actually allow you to reverse the direction of an already-placed underground belt without destroying it and placing it in the opposite direction.
Unfortunately no, it's not possible to place the outgoing belt first.
What you can do instead is build the belt backwards, then remove both underground pieces and add the ongoing one in the proper spot, followed by the outgoing one.
If you try to remove only one underground piece and replace it, the game will match it up with the existing piece (highlighted with arrows when you hold it before placing) and not rotate it at all.