If you book together the systems of the Railcompanies try to provide you with seats next to each other (Often you even can select the seats from a Seatmap)
Worstcase you can still ask other travelers if they would change so that you can sit next to each other :)

So: an utter surprise, we tell that dozens of times daily: do NOT use that app-use the normal other means that travellers use in such cases: mostly the local railways. See overview also by seewulf

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

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

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice.