Okay, here is the catch to me.  One of the things that is to be avoided is building in too much shrinkage stress which is what you will get if you weld the buttering passes with the backing in place and bond to it from both sides.  Now, if you put the backing in place but don't tie in to both sides and the resultant shrinkage does not pull the parts away from each other so you end up with a gap between the backing and the other member. AND, the backing was not attached to the other member even by tacking, You could butter up from one side only until the root opening was at the specified parameters and then let your parts cool before beginning your welding passes that would actually tie both members together.

You either need to butter without the backing to close the opening or proceed as described above. 

Relating this to your original question, I still think you need a separate procedure to do so.  And, I still need to check a thought out that keeps nagging at me about proceeding with the repair.

Have a Great Day,  Brent