I play holy paladin for my 10man group and I've 2-healed every fight with a disc priest perfectly fine. The double absorbs is perfectly viable since this tier has a lot of constant raid damage, the bubbles are very effective. And like Bonkura said, we provide the burst healing that disc lacks with HA, Wings, GoAK and DF if you want to count that too. Going into heroics, our disc priest is going to be shadow ms and I've started 2-healing with a resto shammy and that worked out perfectly fine. Holy paladin's mana regen is still shit imo and mana tide totem is amazing, and the shadow priest can still hymn of hope too whereas a shammy can't mana tide while ele or enh. But, imo I think rdruids would be the best choice beside a pally healer.
Maybe it's cuz I haven't ran into a good disc priest yet but I haven't found one that can beat me while 2 healing so far
It depends a lot on the fight, Discs cap out on their maximum healing quite quickly because they can do so much of it as absorbs, if theres enough damage to overrun that then you'll quite easily be able to beat them. On a more controlled fight with low damage, or farm content, a Disc could potentially zero the healing needed.