I've had another revelation. The fun - and success - that can be had from healing as a team rather than as individuals.
Going from healing 5-mans to 10-man did require some co-ordination but not a huge amount and not at a very detailed level. We were allocated a tank each and played whack-a-mole with the rest of the raid. Apart from agreeing to try to always keep HoTs on both tanks we pretty much left each other to it. Of course, if you see that the other healer's tank is in trouble and you have the spare capacity - you help out, that goes without saying, I hope.
For certain fights, like Curator, we changed the healing assigments slightly (one healer for tank and bolt-soaker, one healer for all others), but again each healer did their own thing to fulfill their healing assignment. I'm not saying that one should have to tell each healer exactly what spells to use and micro-manage them, but what I did find was how much it can increase your success by discussing the tactics with the other healer and agreeing on how to pool your resources in the best way.
I was healing together with my husband's new alt, a holy priest, and as we sit next to each other we can easily chat about the healing side of things without having to go via in-game voice-chat. We both use Grid with the ability to see incoming heals and tracking each other's HoTs, and that in combination with direct communication just made it so much easier. It was also more enjoyable when you felt more as part of a little sub-team rather than two players with the same role in the larger team.
I doubt it made a huge difference to the outcome of most of the fights, but when we came across Prince it made ALL the difference. It was our first try ever on Prince and since it had been a spontaneous on-the-fly decision to give it a go, most of us didn't know the fight at all - though luckily some people had done it with other chars in other guilds. We managed two credible attempts, but on the first one both healers ran out of mana far too soon and on the second we had an unfortunate encounter with an infernal.
For the third attempt me and my co-healer discussed how we had healed previously and agreed on a specific rotation of flash and greater heals to help our combined mana pool to last longer. There was also a couple of shout-outs to each other in the fight "I've got this for a bit, you stand idle for a short while and regen". And we managed to stay the course, keeping the tank AND the rest of the raid up throughout the encounter. That was such a buzzy feeling - we got the job done and we did it as a team!
I am convinced that this tactical chat before the 3rd encounter made a big difference as we then went in focussing not just on our own mana pools, but how to best keep our joint pool up enough to last the encounter. It almost made me want to do 25-mans, just for the buzz of being part of a proper-sized heal team. :-)