The app won’t ask you specifically, but you can specify which dice you roll before rolling by using roll expressions (the button between the dice bag and the X buttons).
You can enter your expression like this for two simultaneous rolls:
5d6, 7d6
Or if tunnels and trolls uses opposed rolling, you could subtract one from the other like this:
5d6 - 7d6
You can view full documentation for notation that expressions can use here.