As Dave says.
You effectively work out the 'bounce' before rolling to-hit, and the bounce can cause up to 1 hit on each stand the line passes through. So a unit in a column of three stands, being shot by a single cannon, will have 2 dice rolled against it for the target stand, and one die for each of the other stands, for a total of 4 dice.
If shot at by 2 cannons, you'd roll 8 dice against this unit.
Basically, the two shots you roll for, per cannon stand, is an abstraction (like everything in Warmaster). Think of it as a rain of many cannon balls, with overshot being frequent. If the unit is in a deep formation this overshot will still hit the same unit, or if another unit is behind it that other unit may also be hit by the overshot.
I don't think it's stated in the rules, but I'd give the cannon a full 5cm bounce even if this does hit things greater than 60cm away (though you must still target a stand within the 60cm range).