My wife and I share a VW Tiguan and this is a constant annoyance. To switch devices you have to go out of CarPlay -> Settings -> Swipe left to the second screen of controls -> Mobile Device Settings -> Select device.

And since the device it initially chooses to connect to sometimes takes 30 seconds or so to be fully connected (during which time you can't make any selections in this menu), you have to sit and wait until it connects the device you don't want to be connected before you can switch to the one you do want. And then you wait about 30 seconds for the correct device to connect.