I have not seen "many" logs of guilds fielding two mistweaver and i am somewhat curious about your thoughts on the topic of bringing two.
My guilds current heal setup is 2 Paladins 1 Shaman 1 Priest 1 Druid and 1 Monk (me), and i am asked whether i would recommend another monkheal.
My own thoughts on this are that i do not really see that much going for us except awesome aoe output. No noteworthy raidcooldowns in 25 man.
I tend to say that almost any other healer would be better on 90% of the upcoming heroic fights (my guild got non heroic content clear) but that might be because my view is shrouded by the prospect of having another monkheal next to me.
If my opinion were purely based on AoE healing alone, sure, why not? But as you said, and as I have also experienced, our raid CD's are pretty bad for 25 man. It's sad going from 25 man to 10 man (some of our people were not hitting the numbers we were pushing them for, so we switched to 3 ten mans), and Revival actually helps a hell of a lot more in 10 man.
To be honest, I would stick with the heal comp you have now, it's pretty good as it is, both in variety of heals, raid CD's, and token distribution (even though our set bonuses are pretty bad for anyone who never uses SuM and EM).