To use double attacks, I guess you mean, you have to have each of the characters with the right ammount of stocks, and then when it's one character's turn and you want to use a double attack that involves him or her you have to boost the other character, then the double option in battle menu will be unlocked.

And to beat Albedo in momo's mind you cain wait until his arms are destroyed and then his break guard will be off and you can break him and do up/down combos to kill him.