In the last release you can only put one on. (At least the "capacity tab" disapears for me I think that's intentional as it doesn't disappear when adding colony or troop modules) |
You only get the benefit of using a single construction module as, like Kid Charming says, the entire ship is used up to build a new starbase/module.
The reason why you can fit multiple colony/troop modules is that you can create a single ship able to carry more colonists to their new home (colony starts with a pop boost) or to carry a larger army into an invasion.
Personally I always design a troop transport with two troop modules and better engines as soon as my tech level allows it as then you need fewer transports to invade larger colonies.