If any of them have a triggered ability that triggers whenever a creature enters the battlefield, they'll trigger for one another
Meanwhile, Mega Gardevoir ex feels like a bulked-up version of the already dominant Gardevoir ex, with moves that push Psychic Energy cards to Benched Pokemon and a Mega Symphonia attack that deals 50 damage for every Psychic Energy attached to a players Pokemon
It will do 60 damage for every energy card attached to the opposing Pokemon, which means that, if they have 4 energy cards, this attack will deal a sizable 240 damage
For many players, this set marks the boundary between the old-school slower Yu-Gi-Oh, and the fast-paced combo-focused Yu-Gi-Oh we know and love today
In addition, both of its attacks are very potent and without any significant drawbacks that render the card less useful in battle