The adapter itself won’t make this any simpler (all it is really doing is mimicking a phone that’s plugged in). However, you should be able to do what you’re wanting as long as all you want to do is answer a call on the secondary phone and don’t care if it’s connected to
CarPlay.
In your case, you would have your primary phone connected to
CarPlay (either wired or with the adapter). Make sure you delete that device out of the Bluetooth settings on your car (or at least move it down the priority list). Then, make sure your secondary phone is connected to Bluetooth and is the primary device (top priority).
With this setup, you should be able to receive a call to either phone and it will take over your vehicle‘s system.
Note I haven’t actually tried this myself, so I’m just operating on assumptions. I can give it a try myself later tonight when I get home (or you can try it yourself, too).
Also note that if you want your secondary phone to be connected to
CarPlay, there isn’t going to be a good way to do it period, especially to accept an incoming call. Wired, you have to unplug the current phone and plug in the new one. With the adapter, the best way to disconnect a connected phone is to turn off WiFi on that device, which will disconnect it from
CarPlay and then it will send you back to the dongle’s start screen, where you could choose your secondary phone. Then you’d have to wait for it to connect. Obviously not something you’d want to do when receiving a call.