Personally I would keep 2 heavys, but it depends on which one.

My two active heavys are Cerys who has the massive damage reduction for pretty long duration (not invulnerability like Mouse mind you), which I use as the opening leader with Victoria in tow as siege TD.

Next I have Alyona who is the support Heavy. I keep her in reserves when I need to reinforce a position that requires a tank that can withstand.
Movenemt emblem and her own active ability are good enough for that. Though I have been considering swapping her in for something else, so far she has performed well and fits my style of running multiple fronts.

Personally I am not picking up Katyusha. I dislike Minnie already to a point. The AI for this type of tank is super wonky, and her "setup" animation take so long you often depleate 1/4 of her active, simply by the fact that the first target dies and she needs to go through the whole fckn animation to redirect. She is amazing when she works, and sucks balls when she doesnt. And the sheer preparation for this takes to much effort and can backfire rather quickly (many times she has been killed by random AOE bs and insta gibbed before even using her one trick pony of a power).
Having two of these, while nice on paper, would be to much of hassle for me to execute.
I´d much rather wait and pick up one of the premium Artillery tanks, or a TD/Medium that has good siege skill like Catriona and Victoria.