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Ok this is a good question, but I think I see where your question can be aswered in D1.2, but you didn't specify a code. Here is the answer for D1.2. First off, you'd go to 3.12.3 where it says, more than one WPS may be used in a single joint, provided each WPS is qualified either seperatly or in combonation with the other WPS's within the thickness limits in table 3.4 or 3.5. That being said you can do a disimilar metal WPS and PQR ,but you'd only be qaulified for that joint with those 2 metals and filler. You have to do a seperate one for 6061 to 6061 and 5052 to 5052 with whatever filler you want to use for those. Your tensiles for the disimilar metals only needs to meet the min tensile of the lower strength base matl (3.7.3.2). Yeah , you don't get to many freebee's with D1.2 cause pretty much anything you do needs a PQR with it almost.