ASCP constraint planning for contract manufacturing environment

I have brainteaser for ASCP guru.

Following is our case:

1. In our environment we own engineering process, but outsource manufacturing to China
2. We buy subassembly item SA from Vendor #1
3. We forecast item SA and cut PO to vendor #1
4. Vendor #1 dropships item SA to vendor #2 location (virtual org in our EBS environment)
5. Vendor #2 can manufacture 2 different finished goods item FG1 and item FG2 using subassembly item SA (BOM for FG1 has subassembly as component; same for FG2)
6. We forecast finished goods items FG1 and FG2 and because both are BUY items we cut PO to vendor #2
7. Our problem is that currently requisitions for both finished goods items FG1 and FG2 are NOT constrained by available inventory of subassembly item SA, so vendor is always altering our PO with availability constraint (for example with response: I have only 1000qty of item SA and your PO demand is 1000 qty for item FG1 and 500 qty of item FG2. I’m capable to deliver 1000 qty of item FG1 and cancel FG2; or 500 qty of item FG1 and 500 qty of item FG2)