My personal opinion, based on what I find works best for me (so take it as such), is to get on with the revisions. Some people take a break between rough draft and revisions, just to let it "ferment." This is not something I tend to do, but if if one chooses to do so, it's a good time to get some of the preliminary work done on the new project. Once revisions are begun, however, I find it best to work through the piece and get it in the best form possible without the distractions of bringing in another piece. In fact, I think revisions are so important, it is essential (for me) to give them my full attention. Once work number one goes to trusted beta readers, that's another good time to get on with work number two. Once comments come back on number one, it's time for me to be single minded about it again so I can get it into shape for submissions. Once the submission game is begun, it's back to work number two.
The most important thing is to find what works best for you, regardless of what others do or say. My advice, such as it is--do not short change the revision phase. It's the make-or-break part of the writing process.