I'm not convinced that my contractor is well versed with double lock installations and I'm concerned whether the correct clips are being used. In order to allow for movement, is there a requirement for using two piece clips anytime you utilize a double lock or is the use of the two piece clips dependent upon certain factors i.e. material type and panel length. In other words, are there circumstances where one piece clips can be used with a double lock installation? Thanks you in advance for your help!