Which Route are you talking about? 

Best way would via the Website of the train company directly (if possible) or via a International Ticket office 

I’ll second this!

As for applicable countries, it’s often the ones where reservations are non-essential (and not automatically included in the ticket price) in which it is easiest to make them separately from ticket purchase; if I remember correctly the CD (Czechia), DB (Germany), OEBB (Austria) and DSB (Denmark) websites all have facility for straightforward reservation-only bookings, and there may well also be others. The UK is also one of these countries, but reservations can only be made in person at stations (free) most of the time.

It’s also fairly simple to make one passholder reservation and one ticket purchase (inc. reservation) in one booking for Sweden on the SJ website, by adding an Interrail pass to one of the two individuals travelling from a drop-down menu. SNCF (France) was also like this until the option to add an Interrail pass mysteriously vanished recently, but it will hopefully return post-pandemic.

i travel from Amsterdam to Perpignan (France).

Reservation 1 interrail and 1 normal ticket (2 persons)

thanks

Ok, well as I say, annoyingly it’s not possible to do this in one online booking with SNCF at the moment, but maybe if you took this journey next year the feature might’ve been put back by then. Unless you’re going by TER (in which case reservations are impossible anyway) you’ll be travelling to Perpignan from Paris by direct TGV as there aren’t any direct trains from the Low Countries.

The Amsterdam to Paris section (I assume you’d go by direct Thalys) is unfortunately also inconvenient online. NS International has, like SNCF, removed the option to add a pass from their website, and while NMBS/SNCB still has it, it’s on a separate booking process to full ticket sales for non-passholders, and - crucially - does not allow you to pick a numbered seat from a train layout, only to register a preference (i.e. window seat, aisle, etc.), so you probably wouldn’t end up seated together.

What I would do if I were you is (and I’m assuming you’re Dutch), just go down to your nearest large station and try to book at a ticket counter. Both a reservation and a ticket should be obtainable, and even if you can’t choose exact seats, you will be put together when you book at the same time and in the same place. I’ve just checked, and Paris-Perpignan is bookable online with NS, so I expect they will still be able (in person) to make Interrail reservations as well.

Don’t forget that most of the time in Europe, booking systems are reasonably interconnected - compared to in the UK, where (in true Brexit spirit) no international tickets or reservations can be booked AT ALL, you’re quite fortunate!