Each one adds its own pass on their Rail Planner app.

Now seat reservations are a totally different thing : you need to book as a single book to ensure you sit together. This can be done on interrail.eu but there are frequent bugs + 2€ fee is added per person per train (it quickly adds up!). Best book through the railway companies or Rail Europe, following this guide : https://www.seat61.com/interrail-and-eurail-reservations.htm

Questions welcome, mentioning route and travel date. :)