Can you record the BV's separately on the S910? The problem with laying all this down at the same time is, you goof EITHER part, you are back to square one.

Personally, I would still record the Arranger .Wav's first in the S910, transfer to the computer, and overdub BV's and mix there.

Alternately, TBH, if the audio interface is already set up for the computer DAW, recording the sequence through the USB interface into the computer initially will negate the need to transfer all the .WAV's off the stick.

Lastly, for the ultimate in sound, might I suggest that you use the computer DAW to actually play the SMF into the arranger? There is one strength (several, really) from doing this. Firstly, if you have either other keyboards or VSTi's with better sounds for certain sounds (drums, for instance), you can play those parts through the other keyboard, or route it through a VSTi. Secondly, if you have the time, you can record each sound separately, or at least do submixes (drums, bass, comps, strings, etc.) and then apply better effects and compression independently to each part. You can only do so much within the arranger itself. Better reverbs, drum room convolution impulses, bass amp simulators and things like this can do miracles to fatten up and make realistic some weaker arranger sounds.

Obviously, if you are on a tight timeline, you are best served doing it quick and dirty, but if this is a labor of love, taking your time and multitracking can give you some amazing results.