Solution to subinventory level plan

Hi,

I have below business requirement:

In one particular inventory org ,there are two subinventories in which one is considered as Customer and another is SUpply

SPARES- Customer

STOCK- Supply

For every SPARES Sales Order, in case of no onhand qty available, item needs to be provided from STOCK.

In Current system , MIN Max planning is creating IR -ISO in Supply Subinvnetory (STOCK) for any Demand (Sales ORder) from SPARES.

Daily , MIn Max program will generate IRs in case of less qty available.

So users are facing the issue that they cant link Spares Sales Order with Stock ISO as requisition table doesnt populate any link as Min Max is purely used for maintaining Safety Stock.