Ami missing something?

Why would you use 1 beam when 2 does like 15% more damage with as far as I can tell, everything else being the same? Like I understand 3+ because it takes more time to cast, you have to hit multiple in a row, etc but 2 beams seems to function the same as 1 while doing more damage. The only thing I could think of is that it’s slightly slower or doesn’t go as far, but if that’s true then I haven’t noticed it in action.