The system will assign seats next to each other.

If for whatever reason that's not the case, you can change them later for free on eurostar.com under "Manage my booking".

Sorry I’m new to the system and I don’t understand yet how it works exactly. In order to be seated next to each other, does one person need to buy the two Interrail passes or can we do it even if we buy ours separately.

I don’t know if my question makes sense...

Passes can be purchased separately but then it’s usually best if one person books seat reservations for the whole group, yes. Create one account on interrail.eu, enter both names/pass numbers and there you go.

Alternatively book elsewhere when possible: interrail.eu adds extra fees. Use Rail Europe for Eurostar and GWR/LNER for the UK. Note that seat reservations within the UK are always optional and free of charge. Do not pay for them!

I’d recommend reading https://www.seat61.com/interrail-and-eurail-reservations.htm

Thank you very much for your precious advice!