Backflushing from multiple fixed locations

I don’t think there is any easy way to do what you want, especially using backflush.  The shop orders are going to pick up the default location for the raw material which can have only one location for the PICK group type.  I think if you want this level of control, you are going to have to manual issue from the intended location - selecting it manually.  Beyond that you would have to programmatically use a custom field on the Shop Order material lines to specify the desired location, then use a different custom RMB functionality to do your own version of backflush that would choose the custom location field first if it is populated, then if not, would use the default location.

Nothing out of the box that will do what you want that I’m aware of.

You should connect Location 1 as an Inbound location to the work centers representing machine A, B and C. And location 2 and inbound for work centers D, E and F. The material must of course be connected to an operation planned on the relevant work center. 

Backflush logic will follow a hierarchy where the location defined on the material line comes first, but if no location is defined here, logic will continue to the inbound locations of the production line and work center. Then any shop floor location will be used and finally any picking location. 

You can also setup the work center to only allow the Backflush logic to use the inbound locations. I.e. When reporting an operation on work center A, and the required quantity is not available on Location 1, system will give an error rather than Backflush from Location 2.