I have found out how to make reservations together with a travelling partner via this link https://www.eurail.com/en/help/reservations/how-do-i-make-seat-reservations-together
but does this work if one has a pass from the UK and one has a pass from Germany?
Yes it works : reservations are a totally separate process, they're only tied to the traveller's name.
Whenever asked oboard trains, everyone shows its own pass and on top of that the reservations PDF.
Have a look at this guide too : https://www.seat61.com/interrail-and-eurail-reservations.htm
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