The reason for bringing two discipline priests is really quite simple for 25HC LK... you need shields on everyone as much as possible, and one disci priest simply can't do that due to the high damage breaking shields early. There's also room for error and emergency response with two disci priests. I've shielded my half of the raid, got weakened soul up, and suddenly someone goes to low health. I can penance them without worrying about falling behind in shielding because I simply can't shield my half of the raid for another few seconds.
Healers are all about playing it safe; that's what our job is. OP, you need to explain to your GM the benefits of having pre-emptive healing/absorbs, rather than reactive healing which has the potential of being too slow. Not only do you weaken incoming damage by effectively giving everyone in the raid nearly 50% more health, you can do so on lots of people because the nature of your absorbs means you can prepare for damage, something the other healers can't do nearly as well. If your GM can not comprehend what a benefit this is, then they honestly should not be GM, OR, they should not have any say in raids, and instead delegate raid-leading to someone who has a clue.