When calculating type advantages, it's always the type of the move used vs the type(s) of the defending Pokemon. As such, it doesn't matter whether the attacking Pokemon is dual type or not (except for whether Same Type Attack Bonus is involved).
The maximum bonus an attack could have is when A pokemon uses a move of the same type, against a Pokemon double weak, for x1.25x1.25x1.25 bonus, example:
Onix uses Stone Edge against Charizard
Onix is Ground/Rock
Stone Edge is Rock -> type match, x1.25
Charizard is Flying/Fire
Rock is very effective against Flying -> x1.25
Rock is very effective against Fire -> x1.25
x1.25x1.25x1.25 = 1.95, so Onix's Stone Edge does almost double damage versus an attack without all the bonuses.
Hope that helps!
