With extender, usually the second network has same name with a _2 extension, and requires same password. Some allow to rename or change password to something else.
Since home audio installations typically are stationary (the components are not moving around the house), the two-network thing has been an absolute non-issue for me. Set it and forget it.
If you use phone to control, pretty simple to switch wifi networks. Takes 2 seconds. Some (I think, don't quote me on this) will work across networks.
I feel like I've controlled my dac/streamers in two separate locations with my phone, where one of those dacs using main network and one using extended network, without switching networks on phone, but I'll have to test that to be sure. Either way, easy.