Currently, when one enables multiple branches and multiple warehouse : it is mandatory to link a warehouse to a branch and no warehouse could be associated to more than 1 branch. Effectively, what it does to prevent from changing warehouse for a line-item.  Only the default warehouse stays selected and if a material needs to be picked from another warehouse, it can't be done unless a transfer order is created. 

Not only this is time-consuming but also highly counter productive. 

Ideally, it shall allow to select appropriate warehouse so specific packages could be created and shipped from that particular warehouse even when being dispatched in the same vehicle.