My rule of thumb with my group is "mechanics first, DPS second". You won't even get to enrage if everyone is dying left, right, and center to mechanics. However, with understanding of the mechanics you'll also have an understanding of the best ways to optimize for DPS and healing and coordinating mechanics in such a way to minimize DPS loss as best as you can.
In the case of your healers, I would look first at how much they're both healing for and how much of it is overheal and how much healing is split between the two as well. There are two important things here: (1) making sure both healers are comfortable doing what they're doing and (2) ensuring there isn't wasted GCDs on overhealing so both can focus more attention to DPS if you need that extra umph to hit the DPS check. It's important to work towards both healer's comfort level. While it's more optimized to have both DPS hard and heal equally, it might not be the most beneficial way to run your particular pair of healers.
For example, in my static the AST I'm partnered with would rather heal more than DPS. This leads us to have more of a 40-45% split of me healing and 55-60% of them healing which allows me to focus more DPS uptime which in turn also means our DPS difference is fairly high - especially when you compare WHM personal DPS versus AST. What's important for me here is this works for us, we're both comfortable, and we've made it to O4S and going through that too.
What I'm getting at is you don't necessarily have to tell your healers "Man up and git gud" but you may have to coach them to work together and see what they can do together and optimize together. Minimize overheal and maximize healing CDs to meet the healing checks then spend the rest of the time burning GCDs for DPS to help the group meet the DPS check. You can still work within the confines of the WHM healing > SCH healing, you just need to minimize "waste" as best as you can.