Speaking in merits iv no first hand experience, but my friend always shouts at me when i get a RDM not a DNC, cos he rekons the best PT he has ever been in had a dnc main healing for merits, so it must be possible.
However @ 55 sync i had a dual DNC pt, not a solo DNC one. It was PLD, BRD, DNC, DNC, SAM, DRG. Odly, althought i only had refresh from sanction and my neck, i held hate just with flash and voke, so id suggest NIN tanking wud have been even better; we had chains well above 10 and were cruising nicely at the 12¬14k mark per hour.
Near the end of the mobs, sometimes one of the DNC's wud get hate, just cos only doing 1 flash and a voke every 30 secs, DNC's healing pulled hate and a few times did get tickle'd. This was obviously countered by having 2 DNC's.
I know 2 seems like alot, but if you imagine, both DNC's healed and both used eva down steps instantly on the mob, they were at a lvl5 daze within seconds leaving the brd to do 2x minuet, with haste samba i think we had, the PT was flying. even on PLD i was putting out ok DoT numbers.
In a merit id say with enuf emnity minus merits, and get people to either /SAM 100% using seigan or /NIN and maybe a DRG/BLU back up, u shud be fine.
GL in ur PT's.