Mouseferatu
Hero
I think you should check again the multiclass rules.
I don't remember them exactly, and I've seen only those in playtest which might have changed... but there was specifically an exception for Extra Attack, in order for multiclass martial classes to get the second attack. Something like "add your levels in all classes granting Extra Attack, to determine when you get your second attack".
If that was in a playtest, it's definitely changed. The rules in the PHB specifically call out extra attacks as a feature that is not additive. (Doesn't stack, in old edition parlance.)