There's no set rotation for Healer's per say but there are optimal ways to spend GCDs during burst windows where the party as a whole is bringing out their biggest guns to optimize their damage that also helps increase the damage of party members as well, so you want to try to do your best things in that period.

In terms of maintaining good DPS, as a player you have to understand how each fight unfolds so you know at which moments you will need to heal and thus also understand which places also give you the best opportunities to DPS. There is also a level of trust you have to have with your party and especially your co-healer partner. If your party members are taking avoidable damage, that's less GCDs you have to spend on DPS.

Once the group has hashed out the mechanical details of the fight, the next step is to communicate with your healer. If both healers are responding to the same damage with healing that results in overheal, that is lost opportunities for DPS from either of you. Likewise, both healers should be communicating so you both know which one of you is going to be using your potent abilities to heal through certain periods of pain, or if both of you have to get involved to maximum effect. This communication will enable you to determine when you can DPS and when you should be focusing on heals. In some cases you'll both need to be firing healing from all cylinders and there's nothing wrong with that.

Once you, your co-healer, and your raid have worked out the mechanic kinks and healing responsibilities, your general DPS thoughts will work on a two point priority system:
  1. Apply / Reapply DoTs (unless boss is about to go invuln - then just keep nuking)
  2. Use your direct damage spells

The end goal for highly optimal DPS for the healers is to reduce any healing GCDs and rely on your oGCD and abilities to help carry the healing and then maximize DPS GCDs. Mileage may very depending on your comfort level and how bleeding edge you and your group want to push the risk factor but this should at least get you started.

Each group may do healing differently too so find a strategy that works well for your group. For example in my group, my AST partner wants to do the main bulk of the healing and this allows me to invest heavily into DPSing as a WHM. You may not have that luxury but you won't know until you communicate with your co-healer.

You also may hear about the "main healer" and "off healer" strategy. This is non-optimal from a pure-healer-DPS standpoint but it's easy to communicate as well so it might be something to employ. Similar to my example above, essentially one healer takes the bulk of the healing with the other providing heavy DPS and some mitigation / healing during periods of heavy raid damage.

If you want any help planning out the fight, there is a bunch of neat time stamp pictures that can found in this imgur album.