Without a doubt the best counter to double manticore teams are those with at least 2 converter troops.
Green sear + giant spider + Mercy (but not exactly necessary, what you want is an empowered troop at least)
Mercy + Alchemist + hellcat (use Gard Avatar)
notes: without traits this team easily kills double Manticore teams even if they drain ever single turn. How? Simple, they can only ever cast one drain a turn, making it very easy to set up both converters full of mana AND casting Gard prior to passing the turn. U can also use mercy as drain bait for the manticores if the other parts don’t work. Also let them take skulls a few times and it is game over, 3 casts of Gard is guaranteed team wipe.