Yveltal have 15000HP and Mega Rayquaza have 20000HP (Elite Raid)
so Dragons Move should be the necesary Challenge to beat Duo Mega Rayquaza with Dragons

Neutral Damage Yveltal is like Super Effective Damage Mega Rayquaza

No Weather Boost is a extra Challenge

in my Opinion i should to Duo Yveltal with Gust/Dark Pulse atleast of Snarl/Hurricane is make way Harder like Mega Rayquaza having Air Slash/Aerial Ace.