True, though I do think it's worth looking for connections between the two. A rail trail running through an area could form a good route for many different short commutes all along its length. If it's all rural, then sure - there's unlikely to be any overlap. If it connects populated areas, then subsections are likely to be multi-purpose.

That's the case for a lot of the trails in my state - there are big, rural, rail trail loops, but the suburban/urban connections to the routes are excellent commuting corridors.