The broadcast rules are that one of the two Sunday networks gets the double-header and the other gets a single game. In any given market, the local game will always be one of those three games, so yes, if the other network has a game at that time, you can switch to the other game.
What I'm talking about is the market blackout rule that says the local game must be shown and can only be available through the network. For example, you can't watch the local game on the Sunday Ticket, you have to go to the network. Similarly, if you're on a streaming service like Verizon, you can only watch the local game.
It's called a "regional blackout".
https://cordcutting.com/blog/what-a...at are broadcast on,all outside of that area.
https://blog.jipel.law.nyu.edu/2017/03/sports-blackout-rules-in-the-age-of-cord-cutting/