Most BT headsets can pair with 4 or more devices. They will usually automatically CONNECT to the last device they connected to, or any device they are partnered with if it is the only one in range and they are not actively connected to some other device. If that's not the device you're currently using, you may need to tell the device to connect to the headset.

Pairing just means the two devices know about each other, and the user has correctly authenticated the two devices, so they can connect in the future without authentication, and possibly automatically.

How they connect (automatic or otherwise, and in the presence of multiple paired devices or not) is up to each device manufacturer.

I don't think any bluetooth device can be connected to two other devices simultaneously except in PAN network mode.