just ask on the train if someone will change with you so that you can travel together.

Why are you sure the trains arent together? Seatnumbers on trains are sometimes confuseing For example i had once 54,56 next to each other :)

If you both booked separately: don't do that if you want to sit together. Book 2 seats in one transaction, best not via Interrail but using other websites:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

J’ai acheté les places reservations pour moi et ma femme. Mais dans le voyage on a mis l’un dans une voiture et l’autre dans une autre voiture. Savez-vous comment reparer cela? Merci

For which train (route, date, departure time)?

Where did you buy them?