Thank you! You just made me realize a situation in which dual CDKs would be better than dual resist leaders. I ran a dual CDK team once, just for fun, because I have a couple CDKs on my friend list. It was cool playing with 40,000HP, but it didn't seem practical over using dual ADK for speed or dual resist for the tough special dungeon bosses. But it's perfect against Divine masks and Sprites that attack in multiple colors which renders resistance useless. And you also don't have to worry about monsters having the same timers either, since you have plenty of HP to survive a multi-hit attack. It's even more synergistic in conjunction with Lilith, since she's also a Dark monster. Her poison skill is a perfect safety net if you do get close to death. And since she's a healer, she'll help you recover HP between attacks. And CDK's change Light orbs to Dark skill will help if you ever run out of Dark orbs on screen. On paper, this probably sounds like the most fail-safe way to complete those dungeons. No having to worry about accidentally clearing a round before Shiva is charged-up, or accidentally slipping a finger and not making a match, or simply running out of the required orbs to kill everything in 1 turn. And like you said, Rainbow Keepers work perfectly in this team, too. It's almost too convenient.
edit: Blegh ... all that excitement made me forget that CDK's leader skill only works for Dragons. Guess it could still work, but not crazy-well as I hastily envisioned.
edit 2: Duh! There are Leader skills which double HP for Dark Monsters instead of Dragons! Strategy still alive! Though, I'm not sure which monsters have double Dark HP leader skills.
edit 3: Oh! It's Thanatos!
http://puzzle-dragons-global.wikia.com/wiki/Thanatos
Too bad no one ever leads with Thanatos. =p