In the past, I have worked on more than one project at the same time. Quite often, in fact. I have
One thing that I've never done (with any success) is trying to write two novels at the same time.
James Dickey often worked on many projects at once. In the days before word processors, he would keep typewriters in several rooms in his house, each with a different poem on the roller.
A couple of times over the years I have tried to work on two novels at once, but what has generally happened is that one overwhelmed the other and I just ended up putting the weaker of the two aside until I could give it the attention it needed. In some cases, I saw this as a true and fatal weakness in the lesser work and killed it off.
The upshot is that I think I'm going to take a stab at working on two novels simultaneously. Both of them are fantasies, but one will be set in my mythical Iron Age world of Mangrove, and the other will be set in modern times. I'll see how it goes. It may work, this time.