CRM ATP check using ECC and External System at same time

Hi Experts,

We have a requirement where we have ECC and External (mainframe) systems as fulfillment systems. We have 2 sets of Item categories one for ECC and one for external system.

Currently, the ATP check works only for one system ECC or External. If ECC is active then External is ignored.

We need some help/pointers how to implement ECC and External ATP check at the same time from CRM system.

Regards,

Vikas