How to plan for two subinventories in Supply planning

Summary:

We have business requirement to plan in MRP plan two subinventories

Content (please ensure you mask any confidential information):

Business requirement: plan for two subinventories using MRP planned items (w/out RP plan)

Our client has same items that are MRP enabled that are used as components for a build and service parts for aftermarket sales. This inventory is kept separate in subinventories, and the customer would like to plan these at subinventory level as well. Since the items are MRP Planning enabled, we can't use in our replenishment plan. So we tried subinventory netting, and a separate MRP plan for that subinv, however we have two separate inventory supplies being drive by same demands. We looked at demand classes, however not sure if this helps distinguish the two supplies downstream in