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I did a couple 7 pound costo butts together and they took 12 hours, plan on 2 hours per pound for just one of the butts and make sure they are not touching each other. If they are done early FTC them. Like Yemtrey said cook to temp and doneness not time, you will know they are done when a probe slides in like Buttah.
I try to stay 250 -275, I find 225 is too hard to hold for lo and slows and with Butts it really doesn't make any difference weather it is 225 or 275.
LG BGE, PSWOO2, Stoker WIFI.