Be able to sync inventory from more than 1 location

We have 3 locations at the moment. Warehouse is where complete boxes are kept, Default is where part boxes are kept, items picked and despatched and FBA where Amazon hold and fulfil orders.

Currently we can only sync inventory for non-FBA channels from Warehouse OR Default not both. This means that when stock reaches zero in the packing area the listings not on Amazon end when we still have stock remaining in Warehouse.