And I'd imagine that you won't be able to join a particular faction without commiting yourself to that side. You'll probably be able to take only one side in each playthrough, at least with the NCR and Ceasar's Legion, seeing as both sides are at war, it wouldn't make much sense to be able to help both sides and still be trusted by both.
As well as it not making any logical sense, the game mechanics wouldn't even let it happen anyway, as killing a town of Ceasar's Legion will probably make them hate you forever due to the Reputation system.
You might be able to play both sides (do small jobs) for a short while, but soon enough you'll have to grow a pair and pick a side. Long live the Legion!