(There are many examples of this in the NBA, I just mention this one because I'm a pistons fan and that's what I'm familiar with.)
Here's the minutes distribution from a pistons game. You can see how they both start, but then minutes are staggered so one of them is always on the floor. So technically Ivey is the back up PG, even though he starts. Same for Hall when he comes back. Hall will start and play the back up PG.
There are a lot of actions that require multiple ball handlers, and both guys play off each other. With the Pisons, you even see Sasser (another PG) share the floor with both Ivey and Cade.
Even against Idaho, there were times Baker was playing with Egor on the floor and Baker was distributing the ball.