Hi Millis ,
Thanks for your post.
You can manage the two inventories of your shops with ATUM Multi-Inventory. It allows you to create as many inventories per product as you need, and assign a distinct stock, price, SKU, etc for each.
You can read all the documentation and features in our Knowledge Base.
However, It will probably not be compatible with your POS plugin. Probably, the stock will always be reduced from the Main Inventory when using the POS (maybe it will do the trick if the Main Inventory is the outlet and you only use the POS for the outlet, but you should try it before buying). You can install a trial from the ATUM -> Addons page.
FYI, we plan to launch the ATUM App this month or at the beginning of October. It will include a POS module compatible with Multi Inventory.
Best Regards,
José Andrés