Well during battle animations you can see that one unit is actually 5 units 2 HP each, so joining units would be putting 2 units together.
But for Bombers, APCs, T-Copters, Ships and Megatanks, there's only one unit with 10 HP, so how do they "join"?
Well, the surplus from combined HPs being over 10 is probably recycled parts being traded for money. Spare foot soldiers probably go back to the barracks or wherever. As for Bombers and whatnot,
-They land or stop
-Parts from one unit are given to the other
-The spares are recycled and traded for money