Can you have two combat squads in the same tranport at the same time? Than have both dismount and remount the vehicle?

I wouldn't think so. After you combat squad them, they become 2 seperate units. I'd check your SM 'dex.

unfortunately not you can only have one squad in a transport.. I will pull out the rule in a few.. let me find the book... And with Combat squads they count as separate units in every way..

The exception to this is the Drop Pod.. While you are coming on to the table.. you are one squad.. but once the hatches pop you then split and stay separate for the rest of the game.. You must decide when the doors pop..

the Space marine codex does say that once combat squaded they count as separate units.. And page 66 of the BRB states that transports can only ever hold one unit plus attached chars...