Thanks Jaideep, an example would be something like this

Product
WHA\Stock: 5 units
WHB\Stock: 3 units
WHC\Stock: 2 units

Sale order:
Assigned Warehouse:  WHA
Qty: 10 units

Expected Outcome of Rule:
WHA: Picking Order: 5 units
WHB: Picking Order: 3 units 
WHC: Picking Order: 2 units
Next the route would push al units towards WHA\Out


This is the onIy thing I managed to achieve:
WHA: Picking Order: 10 units
WHB: Internal transfer request to WHA: 5 units
WHC: Internal transfer request to WHA: 2 Units

There are two problemes with the last workflow. If somehow WHB receives stock, it ends up sending more units than needed and If you packit before sending to WHA, and the qty exceedes the amount of the original order, the pack breaks and dissapear. 

I also tried using an automatic resupply rule, but you can only set it up for a single warehouse or as a "Make-to-Order" (MTO) rule. It also always requests the total lacking quantity instead of looking for available stock in the other warehouses.

A manual request procedure isn't a viable option for us because it doesn't instantly reserve the quantity from the other warehouses.

    

To the best of my understanding (hope am wrong), the functionality / expectation expressed in the post isn’t possible out of the box.

Multi warehouses can be configured in maybe 2 ways

1. Each WH as stand alone - To pick products from different WHs, the routes are configured sequentially, where product qty are reserved in sequence. 

WHA is resupplied by WHB which is resupplied by WHC etc.

With flexible routes in Odoo it is also possible to have products route directly to destination location, rather than the source. 

The fallout is what you describe, operations will be created to fulfill source as well as destination ie WHC to WHB to WHA, while the reserved qty would have WHC to output; WHB to output and WHA to output.

The operations for inter WH transfers would need to be canceled, as the products moves would be direct to destination location. This can be achieved by automated actions, somewhere in the lines of — operations where source and destination are WH/stock locations and have an SO as a source document.

(Believe this is what is referred in your post >>> MTS always creates the picking for the total quantity in each rule, so the sum of the pickings ends up greater than the original quantity.<<<<)

2. Have hierarchy of WHs - your reference here I believe is Virtual WH with the 5 physical WHs as child WHs. Since all WHs are same hierarchy (weighage or sequence), product reservations happen simultaneously across all of them.

A WH could be a child of another WH, so not always necessary to have a Virtual WH. It might be possible to have WHB,C,D as child of WHA

Maybe a sequence and hierarchy.. if not in WHA, reserve from virtual WH which has B,C,D as child.

It isn’t clear if your reference to FIFO is for removal strategy applied within each WH or across all WHs.

To have it applied across all WHs, you would need the hierarchy, if it is limited to within each WH then a resupply route would suffice.

Not sure if this response helps, unfortunately not a short or precise answer.