Mitigation tends to be more important in Savage where the later bosses are more about hard hitting raidwides than frequent damage, so having 2 healers with lots mitigation each is a very nice advantage. Not to mention Sage and SCH both have an ample pure heal and HoT toolkit.

The shield confliction doesn't matter. If both healers are GCD healing, you're playing wrong. Stick the SCH on Succor Duty if you ever need a GCD heal. There might be a case for going AST or WHM for very early prog, but otherwise SCH/SGE should be a very strong comp.

Similarly, AST + WHM will be fine as usual too for most content. For example, any combination of healers can effortlessly heal EX without using GCD heals.