This question is a bit vague since it seems to simply be proposing a solution, but I will do my best to give an explanation.
For the plan to travel to Mars, much of the plan revolves around what is called the space railroad, I think. Now while it is not a literal railroad, the plan calls for an orbiting lunar station, at which, assembly of a much larger rocket would be possible. This concept of assembling in space is a well know, but not commonly executed way of creating spacecraft. Until recently, the need for such large spacecraft has only really been limited to the ISS.
To bring in a semi-realisitic situation, in the video game, Kerbal Space Program, from what I understand, this is common practice for missions to other planets. But, since we have not sent a mission to another terrestrial body other than our moon that required the ability to get back, the need for a heavy spacecraft could be overcome by simply launching a bigger ship from the ground.