Booking via eurail should be the last choice, but here's how to do it:

https://www.eurail.com/en/help/reservations/how-do-i-make-seat-reservations-together

There are better and often cheaper ways to book seats. Check this thread:

And this page:

https://www.seat61.com/interrail-and-eurail-reservations.htm

Reservations are not linked to your pass.

One person can book both the seat reservations. Booking them together will group them together in 99% of the cases, (depending on availability on trains with mandatory reservations).

When booking with some national companies, you can select your seats too.