Check Two IN BU to fulfill an Order

Here is question:

I have two IN BUs that are in the same Distribution Network. Same item is stored in both INBU.

Available Quantity in INBU 1 = 20 EA

Available Quantity in INBU 2 = 30 EA

Par Replenishment Request requests a quantity of 60 EA for the above item.

A MSR gets created and shows the requested quantity of 60 EA.

I run the reservation process and review the Stock Request status.

What I am finding is that it is reserving a quantity of 20 EA in INBU 1 and creating a backorder for a quantity of 40 EA in INBU1 also,.

I was hoping that the system will use the Distribution Network and will fulfill a quantity of 20 EA from INBU 1, then it will fulfill a quantity of