This is a nice arrangement, and it works very well with violin and cello. I think this is done on very good samples rather than real instruments, but the fact that it is hard to say shows that they are indeed good.
I wonder if there could be a little more variety in the arrangement? In the original, the vocal sound cuts through Bach's keyboard figuration, but here the timbres of the two instruments are quite close, so it might be nice to provide a little contrast by letting the cello have the melody for some of the time. Also there could perhaps be a little more dynamic contrast I feel.... most of it sits in the middle of the dynamic range, and yet there is room for some nice crescendi and sudden softs too.