I believe you can, but D1.1 doesn't make it easy for you because most fabrication uses one process for the entire joint.

I would weld the root with GTAW and record the thickness of the deposited weld metal and use it as the basis of determining the maximum thickness that can be welded with GTAW. Then the  SMAW deposit thickness would be determined and that would serve as the basis of determining what the maximum base metal thickness that can be welded with that process. The SMAW would be considered to be welded with backing, whereas the GTAW would be without backing (assuming you are not using any backing while qualifying the procedure.

An alternate means of qualifying the procedure is to do each process separately and then combine the two, GTAW and SMAW, into a single WPS. I prefer the later method.

The rational I cited above is based on ASME Section IX, but it should be valid for AWS D1.1 in the absence of more difinative direction from D1.1.

Ultimately, your customer or the Engineer will accept or reject the PQR and WPS. They have the final say on most projects.

Best regards - Al 6

You won't find an explicit approval of this method. Neither will you find a prohibition. It is an acceptable method as long as the essential variables are complied with.