Disc are not "tank healers" in the classical sense. For that matter, neither is a Paladin.
Both are tanks healer. The underline is important. A Paladin's next-best to healing two tanks is to Beacon the single tank she is assigned to, and snipe heals in the raid. Viable, but hardly doing tankhealing as the main job then, right? Let a Pally not use Beacon (and actually focus on a single tank), and I nom nom them while making them redundant on my Shammy. I got AF, I got more output, faster reactions (I can actually heal most fights in 10man reactively, yes. 1,1s HWs for the win.).
Likewise, a Disc assigned to only 1~2 targets suffers. Disc starts flourishing at something around 4 targets, being able to use Disc Shields like crazy while using Flash Heals and Penance to cushion spikes among the tanks. When we heal Marrowgar25, we use 1 Disc + 1 Shammy for the tanks, Shields + CH, much better than a Pally on two and a Disc on one.
Paladins excel at healing two tanks, or two important targets. But they need to be given the chance to do that as 50% of their healing is based on it. Disc excels at shielding multiple targets which are part of a small set of targets to be healed. They can rarely do all the healing on them alone, but in total it is much better than assigning them one target and another healer the other three. Let both share all four targets.
-- (The Death of Rats, Terry Pratchett, Soul Music)