No, bounties cannot be divided. [From a post on MSE:](https://meta.stackexchange.com/questions/16065/how-does-the-bounty-system-work) 

>### 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.