From the GMA Techline
Subject: Floating interface testing for direct shear Can you give us some guidance on the appropriateness of conducting geosynthetic direct shear testing using “floating” or multiple interfaces? It is our understanding that this method is typically used when the critical interface is not known. (Victoria, USA)
Reply: The first test using multiple interfaces was actually conducted in the bed of a dump truck in about 1985. This was obviously not lab friendly so people have tried to open up the direct shear box to allow for a multiplicity of geosynthetic layers. Problems in so doing are inadequate gripping, folding of individual materials, and lack of reproducibility. That said, ASTM does have a task group working on a draft of the test, but some people have doubts as to a final standard.
Now for the personal bias. I think experienced designers/regulators know the critical interface(s) in a layered liner or cover system. Even for relatively new and inexperienced designers/regulators, there is a lot of published data available in the open literature. Of course, if you are dealing with unknown products it might be a compelling reason, but that is most often not the case.