I do not have access to AWS code books any longer (it's been over 10 months since I've had access to the AWS, API, & ASME codes), but as I recall there is a documented method to address joining materials from different groups in AWS just as there is in ASME's B&PV Code (of course, I could be wrong, it's been ten months since I've opened a code book).
I qualified more ASME procedures than AWS procedures. I had qualified procedures for P1 to P8, P5A to P5B - I also had one for inconel to stainless (P45-P8? I can't remember the P numbers for certain). I think the primary difference betwen the two procedures was the Charpy testing required in AWS that was not required for my ASME procedures (or was it API that required Charpy?)
Have you reviewed the applicable AWS standards? Which AWS code are you using? Do you have a copy of AWS B2.1?