The first ABY box I bought was a cheapo Donner unit off Amazon, and it has the level controls for each channel. They are actually very handy because you can fine tune the volume of each amp at the pedal rather than walking 
all the way over to each amp to adjust the volume. 

 (Obviously that's not really a big deal). Unfortunately that unit does not have isolated outputs, so it makes both amps hum like a sonovabitch.
Anyway, the level controls while handy are really unnecessary.
That's why they offer the Little Dual II as another option
https://lehle.com/EN/Lehle-Little-Dual-II