I sort of answered my own question. Someone asked the
same question on the Nexus Forums. In that thread, he posted a link to a
page in the wiki that I had somehow overlooked.
It appears there is no way to achieve the beam splitting effect as it is described. It can split the beam or increase damage, but not both. I don't understand why that must be the case, but there you have it. At this point I don't really want to leave it untouched because I feel like it's somehow broken. I thought I would enumerate some options, just in case anyone else has any opinions on the topic:
1) Switch the "Beam Splitter" to increase the damage. I thought about making it another +3 bonus to go along with the "Focus Optics" mod. I went with +3 rather than 30%, because the smaller bonus seemed more appropriate when the weapon didn't have the drawback of splitting the beam.
2) Leave the beam splitting effect in place, and increase the damage of the "Focus Optics" to compensate. I would probably increase it to +6 from +3, because I'm not trying to cheat.
3) Leave it alone, since sneak attack criticals with multi-shot weapons are essentially an exploit anyway.
As I find myself writing these out, I think I'm starting to lean toward number three. Any other opinions? I don't think I would bother releasing such a simple mod, I just want other viewpoints.