You CAN do it all on one character, and if I wanted to play an alt job I wouldn't have the fortitude to start from scratch on another char.

That said, you will hit a point in endgame for your DoW/M jobs where you're going to have to choose which one to gear. You could gear multiple jobs at the same rate, but that would be unrealistically slow, so instead you'll have a well-geared job, and a gimped job.