I have a mega gengar(alive) and a blaziken with a blazikenite. Gengar dies, so can I evolve my blaziken now?
No. You can only Mega Evolve once per battle, even if the Mega Evolved Pokemon faints.
Source: My Experience.
Sorry mate but only one mega at a time.
No you can only mega-evolve once per match, if gengar faints as mega gengar then you cant go mega again, even if he is revived he will return as normal gengar
No, there are no exceptions to the Mega Evolution Rules.
One Mega-Evolution per battle.