Having 2 Discs in a Raid never bothered me. There are two tanks, so getting PW:S on one of them is too easy. Now, if there was 3 in a raid, I'd probably be a tad annoyed.
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Also, you shouldn't be getting on the Shadow Priests case, IMO. All he is doing his helping you and being raid smart with keeping an eye on his health. That's one thing that makes shadow priests so nice to have in raids.