Whatever works for you is a good answer. However, what I'll add is, if you work on things simultaneously, progress on each individual work is going to be much slower. If that's a sacrifice you're willing to make, then go for it.
What I do is I concentrate my efforts on one work, but if something else pops up and I just HAVE to write it out, I go ahead and do it. That's how I ended up writing several short stories, and that's how I've gotten started on a second wip novel.
With that said, however, I do not plan to work on both at the same time. At the moment, I'm waiting for an agent to get back to me (She passed on my novel but asked if I was working on anything else. I sent her everything I had.) So basically, whichever work she likes most/thinks has the most potential is the one I will finish first. And if she decides to pass completely...well, then I'll just pick one and roll with it. I'm just not patient enough to inch along on two projects at the same time.
However, if time is not an issue, or if you have a lot of spare time to devote to writing, then maybe it can work for you.