Personally I think wanting to make changes to accommodate sch/sch or even whm/whm is a bad idea. Class diversity is very important to the overall health of a game and making stacking any class viable should be actively fought against. I would much prefer to see more interactions and rewards for coordination between the classes (not just healers) couple of quick examples to illustrate the point:

Trait for Regen that as long as a galvanize shield was on the tank that regen would replenish hp first then the shield value up to the maximum amount the shield was originally created for.
Trait for Succor that if a target is already affected by the Medica II HoT then no healing is done but rather a 250 potency barrier is erected on the target.
Trait for Adlo that if the shield has not been consumed and the target is hit by a cure spell that the duration is refreshed (not the amount of the shield). (could also invert this for stoneskin to have its duration refreshed by physick or embrace)



While I don't expect these to be viable because they are off the top of my head I would really love to see more interactions between the classes, and think this type of design is much more inclusive to the community as a whole.