Every time you load an operating system, that OS needs to load the hardware drivers. Even if you are loading from hibernation, it still loads/refreshes the drivers again. Try it... uninstall a driver in Device Manager, and immediately hibernate. When you restore from hibernation, the driver is there in Device Manager again as if you had restarted.
You need to pair the bluetooth devices after the drivers have been loaded.
The only way to keep the bluetooth devices connected, is to keep the bluetooth receivers active. The only way to keep them active, is to not unload them from the system. The only way to not unload them from the system, is not to shut down the operating system that is accessing them.
Now, if you were running one of the operating systems in a virtual machine, that's a whole different ball game. But since you are not, and since you actually reboot between changes, then no. There is no way with your current usage choices to make the bluetooth devices avoid having to be re-paired each time.