Thank you very much for linking the Black Diamond tests to back me up. And thanks very much responding with the most basic rule of nylon on nylon. Running rope over a sling has nothing to do with this.

If you must join two slings, use the same materials and width Symmetrical knots (like the Strop Bend and Climber’s Hitch) appear to perform better than a standard Girth Hitch when joining two slings together.

It seems their tests show that using a Girth Hitch is not preferable. Their data even shows that using a girth hitch when joining slings caused their slings to fail at around half their rated strength, with 70% of their rated strength being the highest when they used the same material.

When you girth hitch slings together, neither part are moving. It's totally acceptable, and furthermore safe the only real caution is that you need to use two slings of the same type of material, and you should really only do it with nylong as opposed to these new future materials like dyneema.

You didn't say this. You said it's totally fine to girth hitch slings together.

The slings do move. There is absolutely no way you can girth hitch two slings together to take out any tightening that occurs during a fall. This tightening generates friction, nylon on nylon.

Edit:

I'd like to add one final thing. Look at where the slings failed. At the hitch. Why do you suppose that is? Take a guess.