Slitherine

I had a BG of 2 bases of knights, one base in rough terrain causing severe disorder and the BG was disrupted.

IIRC disorder and cohesion are not cumulative, so the dice loss would be:

Base in terrain has 2 dice, so loses 1
Base not in terrain has 2 dice, so loses none.

Not

Base in terrain loses 1 dice, leaving 1
Base not in terrain has 2 dice.
Total for BG is 3 dice, so it has to lose 1 more for disruption.

Is that right?

Yes, the chart on page 92 says, "Dice loss for disruption, fragmentation, disorder or severe disorder is not cumulative, but whichever is worst applies." In your example, you'd lose 1 die for disruption or 1 die for severe disorder of 1 base, so either way you are still rolling 3 dice. Of course, if you were fighting 2 different units, the base in the terrain should only get 1 die as that would be the worst for it.