How can two people listen to audio on an Android phone over Bluetooth? I.e. they have two Bluetooth headsets and want to pair both at the same time?
As in most things, it depends. In this case, since you do not state your device, I will demonstrate from mine, a Samsung 8, running Android 9 (One UI). It explicitly allows dual Bluetooth through settings. If this is your device, click through to it at
Settings > Connections > Bluetooth > Advanced > Dual Audio > ON
The setting allows sound to two different Bluetooth devices.
If your device is different, look for the capability along a similar setting path.
Apart from soldering 1 Bluetooth receiver output to two Bluetooth transmitter inputs, I cannot see a way to do it with the current Android Bluetooth setup. It only allows one device paired at a time. Could be an interesting hardware project if you are into that kind of thing. Your phone connects to the Bluetooth receiver, while your two Bluetooth headsets each connects to one of the transmitters.
If budget is not a constraint, and you just want to connect multiple headsets in a fixed location, check out the Plantronics Savi 7xx or 8xxx line of headsets. They are intended to allow monitoring of telemarketing calls, but we use it when my wife and I want to talk on the same mobile phone call.