What happens if M Gengar EX uses its "Phantom Gate" attack to copy Xerneas EX's "X Blast" attack? Can it use "Phantom Gate" again the following turn at all or not? Or would it just not be able to copy "X Blast" twice in a row?
The X Blast attack puts an effect on Gengar that prohibits using that particular attack next turn. But M Gengar EX is using its Phantom Gate attack on the following turn, so yes it is able to copy X Blast twice in a row.
Phantom Gate