If you're tank healing maximize your use of Serendipity and use it on greater heal to top the tank off. Flash heal on him almost all the time, pop a renew maybe. You'll be pretty much stuck with flash heal x3 then greater heal when tank healing.
Prayer of mending is also very nice, but the disc might replace it. CoH on group when they get some damage and tank is ok. If they're all taking tons of damage, you can do Prayer of Healing on the group with the tanks.
It really is tough going holy/disc, did it once, not the best combo, but it can work if you focus way more on the tanks than the group, while the disc covers the group. I don't remember having so much issues, probably my tanks were better geared than yours.