You shouldn`t need an off tank, we just have MT pick up both. Use a Bubble/sac, pain supression, hymn rotation. Tank should have cooldowns ready for it as well and be using them in conjunction with healer cd`s cause he will be taking a lot of damage. Heroism (or in your case Bloodlust) as it spawns to help.
The more I play on hardmodes 25 man the more I like 51/20/0 though I do have a 51/0/20 build that I use depending on the boss (I pve too much when I have 2 pve holy specs). What are the extra three points they are taking in holy, blessed hands and aura mastery.
If you watch Drama`s video of their 25 man kill its a really good example of how range should spread out, basically a semi loose clump ready to Q or E strafe out of shadow crash, and then move right in on it. A healer on either side close by, so if it comes from them the casters don`t have to run half a zone to dps. For this fight to work you raid shouldn`t be taking much splash damage at all.
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Bode - 100 Holy Paladin - Lightbringer