It is almost always better to work on them one after the other. I am currently bootstrapping my startup through consulting/freelancing. Even though i try to restrict my consulting work to about 20 hrs/week, there is always spillovers. But the biggest problem is that of a context-switch. It is just not easy, at least to me, to quickly switch from a consulting gig to my own startup effort. One of the 2 always suffers.