They constantly overlap! Once I get bored or I'm stuck at one, I continue on the other one. With that, I keep good momentum going and keep myself in the zone. I think one project would not work for me, because if I would get stuck or have a creative block, I would just sit on the couch watching TV.
Also, how do you manage your time between projects? Do they overlap? Do you strictly try to work on 1 at a time?