No, bounties cannot be divided. From a post on MSE:
Can I affect / divide part of my bounty to two (or more) users?
No. If multiple answers to a single question have earned bounties, it's because the question has been subject to multiple bounties.
However, as alluded to in the quote, it would be possible to place a another bounty on the same question. (Note though that the size of the bounty would need to be doubled, if it was not maximal.)
I think your current decision-process is fine. Generally, I think one should not overthink this, but just award it as one finds it appropriate in the moment, possibly explaining the rationale.