Can't split requisition when two supply partners have the same association but other products

While testing OLMIS-5142, I noticed that we were able to assign different partners to the same association, but then we can't split a requisition into two partners.

Reproduction steps:

1. Log in as an administrator.

2. Create a new supply partner with the same association as CHAZ (Program ARV, Supervisory Node CHAZ Lilongwe, Facility Lilongwe Health Center) but with other products e.g. ARV001 and ARV003.

3. Initiate a requisition for Lilongwe Health Center and fill in all products.

4. Go to Requisitions > Approve.

5. Click "View Requisition" next to the new requisition and try to approve.

Result:

• There is no way to approve requisition because the same Supervisory Node is used.

Expected result:

• It should be possible to split the requisition into two partners, even if they have an association with the same facility because they have other products assigned.
  Otherwise, one should not be able to assign the same association to different supply partners.