If your phone allows pairing two bluetooth audio devices, then the Avantree product on each headphones is all you need. I don't know if any phones support that.
Or the Avantree product with a traditional headphone splitter.
They're basically different solutions to the same problem of letting two people listen to the same source via Bluetooth. The nice thing about doing it on the source side is that you can use whatever Bluetooth headphones you like, including ones where the Bluetooth receiver is built into the headphones themselves.
This product is a set of receivers for traditional headphones from a bluetooth audio source.
While they're related (one source to two headphones), they're solving different problems.
If you used the monoprice product with two of https://www.amazon.com/Avantree-Bluetooth-Headphone-Receiver... you'd have a similar setup.
If your phone allows pairing two bluetooth audio devices, then the Avantree product on each headphones is all you need. I don't know if any phones support that.
Or the Avantree product with a traditional headphone splitter.