No, they cannot both be used within a single call to knnsearch.
I don't really see how "merging" would really be that helpful. It seems to me that that would be effectively equivalent to defining some new, different distance metric. If one of the tried and tested distance functions doesn't really do a good job, I don't think a newfangled one will do much better.
You could, in principle, create your own version of knnsearch that does that merging, by copying the built-in function into your own directory, calculating both distances inside your new function, and somehow do the "merging" you have in mind.