I don't know about electric piano, but I used it to connect an electric guitar amplifier to my Bluetooth headphones. It works, so you can hear something, but the latency is so great that there is a very noticeable time lag between the attack of a guitar string and the sound in the headphones. You can also hear that the headphones "restart" with every note.
For me, not suitable for the intended purpose.

Can I pair tx and rx if I use two of them and transmit from a guitar to the amp (or from an FM tuner to the amp both without bluetooth always via jack? And if not what other tx and rx adapter would work? Otherwise it doesn't seem right to end them as tx and rx...

If the electric piano has a 3.5 mm mini-jack headphone output, it should be possible to connect it to a Bluetooth headset/headphones.