Reservations are separate from the pass. You can have your passes on different phones or on the same one, but that doesn’t make a difference for the reservations. But it's usually better to have them on different phones, so you can also travel separately if needed.
As to booking reservations in general:
The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail 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 topic, and you will get advice.
Please note that Interrail charges an extra fee of 2 EUR per person and ticket in addition to the fee for the seat reservation.