What about a spell that only targets Beamsplitter, but does something else as well? Maybe you want to save Beamsplitter and friend from a board wipe or a tricky combat situation with Unexplained Disappearance but you’re worried about that Surveil 1 affecting you. Or maybe you want to throw a Take Heart at Beamsplitter And Friend to gain extra life. Both of these are fine! It doesn’t matter if the spell does stuff to things other than Beamsplitter, so long as Beamsplitter is the only thing being targeted. Well, what if Beamsplitter is targeted more than once? Bounty of Might is the bigger badder version of original Ravnica limited all-star, Seeds of Strength. Because it says “Target creature” 3 different times, you can aim all three of those blurbs at the same thing; this could give three creatures +3/+3 each, or +3/+3 to one and +6/+6 to another, or a hefty and terrifying +9/+9 to one target. And if you go with that last option aimed at Beamsplitter… it will trigger. It’s still an instant or sorcery spell that targets only Beamsplitter! So you’ll be spreading +18/+18 across two of your creatures; that’s gonna hurt.
Do note that Beamsplitter must be the only target period to trigger, not just ‘the only target which is a creature’. Gravitic Punch targets a creature AND a player, so it won’t trigger your spell-sharing Vedalken buddy.