Can you be paired to a bluetooth car speaker and also be heared thorugh speakerphone?

Hopefully the title makes sense. My understanding is that bluetooth on an iphone can only be connected and transmitting through one device at a time. So if you have it set up in two of your cars and your cars were parked next to each other and running you could only be talking to the person on your phone call through one of the cars at a time correct? The other situation that occurred is with speakerphone. If I'm talking on speaker phone with the person on my phone call and someone is in my car (obviously it's running), can they hear my conversation through the bluetooth. Is it possible the iphone connected to the car bluetooth and also I can hear and talk to the person on my phone with speakerphone?

Sorry that's confusing to explain. A friend claimed to have heard my boyfriend and my conversation through the friend's vehicle's bluetooth (they were parked next to each other and my boyfriends phone had previously been paired with that vehicle as well as his own). He can't remember if he was connected through his cars bluetooth or just had me on speakerphone? I didn't think it was possible.