I like the idea of Uku & Lele (or Leilei/Leilie), it's a simple but effective title, it also sounds really fluid. Bongo & Ukulele, while it sounds neat doesn't achieve the same simplicity or fluidness (it's just too long).
The problem lies in the word ukulele which has four syllables (yoo-kuh-ley-lee), which when combined with other words just makes the title longer and less fluid. Just look at banjo-kazooie it has waaay less syllables, which leads to a more fluid sounding title (ban-joh kuh-zoo-ee). That's the reason why I think separating Ukulele in two is the best option.
The boss of all Yoshis