How to pair bluetooth keyboard and mouse to MacBook Pro when the laptop is connected to an iMac that is used as a second display?

I have an iMac and a MacBookPro. I would like to use the laptop with a larger screen so I am connecting the two computers with a Thunderbolt cable and press cmd+F2 on the bluetooth keyboard that is paired with the iMac. Unfortunately, I can't pair the bluetooth mouse and keyboard of the iMac with the MacBook Pro. Perhaps, because I need to pair them with the iMac first so I can log in to that computer and then I'm unable to disconnect them because I would lost control over the iMac so it wouldn't let me do that and they are still paired with the iMac when it is just a second display. Is there a way around this? Thanks!