You literally have to buy the '2 heroes for 30 days' to get it to even do anything.
Though, admittedly, when I clicked 'improve' within the game, it let me "improve" both slots independently.
The whole thing is relatively confusing, but yeah... I hope you get your thing figured out. If you wait for 3 days and nothing happens with support, post in the support technical assistance thread with your ticket number.
I get about a 1 - 2 week turn around time with my tickets, idk.