It would be an interesting change, to say the least. If anything, don't "force" that composition but give the players the option to run an unorthodox composition if they so desire. Just to reiterate on my previous comment, if WHMs can heal enough to cover for both themselves and their co-healer, either have the co-healer go full time DPS or just bring another DPS and go nuts. This would give WHMs a niche compared to say, bringing a SCH who might need to rely on an AST to compliment their kit so they can meet the heal checks - but still have enough heals together to meet said check.