i work almost exclusively on multiple projects, honestly. it actually helps me keep focused and lets me improve my skills as a designer and creator at the same time.
for example, while working on Joylancer, my roommate and i will occasionally take a few days to work on other stuff instead. this keeps us really active and energetic, without us getting bored with the project. a big problem that i personally have is that if i focus too much on a singular project, i start to dislike it and even run into pretty major design humps. by allowing myself to take a break and experiment with other things, i learn more knowledge that i can then apply to another project, such as the "primary" project i'm working on at the time.
to me, it feels like both the most organic and beneficial way to make games... but i also have like, three other jobs outside game development, so maybe i'm just a worker by nature!