They are intended to be attached and it is possible you could encounter a particularly officious ticket examiner who would take issue with them being separate,
In that situation I would just staple it myself, as they need to be presented together and it will just draw attention for yours to look non-standard.
This is what they typically look like, 1 or 2 staples on the extreme left of the pass with the ticket attached above the travel diary section so it can be folded up together without creasing the ticket.
https://rail.cc/wp-content/uploads/sites/2/2017/08/interrail-pass.jpg