Multiple vendors for outside processing on BOM - Saw and Zinc plating

I’m trying to set up a BOM with two subcontract operations for different vendors. I’ve created separate non-stock subcon items with the relevant vendor and added the two operations to the BOM. Each operation has the non-stock item on the material line. I’ve added two Work Centres with Outside Process checked.

Material type=Subcon, Subcon Source=Purchasing

My problem is that when i go to the Outside processing tab, I can select the Vendor for Operation 1, but this also defines the same Vendor for Operation 2. If I amend Operation 2, it then changes Operation 1.

Not sure if I’ve set up incorrectly or if there’s some restriction on selecting multiple subcontract vendors on a BOM. If the latter is the case I could try using a phantom sub-assembly.