Is it allowed to issue one EASA form 1 for multiple different part numbers?

Yes, but under specific circumstances as detailed below.

There can be multiple items on a EASA form-1, but it must be related to a work order. (Both examples below come from a 21G).

Example 1 of 2: Multiple PN’s on an EASA Form-1 (new produced)

Example 2 of 2: Multiple PN’s on an EASA Form-1 (new produced)

For EASA Form-1’s issued by Part-145 organisations it is allowed as well as long as it is also related to the same work order. See example below for details of different part numbers on an EASA Form-1 related to the same work order.

Example 1 of 1: Multiple PN’s on an EASA Form-1 (used parts)

It is not allowed to mix EASA Part-21G produced parts with EASA Part-145 maintained parts on one EASA Form-1.