Just tell yourself that leather is a natural material, and as such there are natural variations in how it all works out.
Having said that, black is black, and unless the supplier has really screwed pooches instead of paying attention to their process from day to day, it will match up very well if not perfectly. Even if there was a small shading difference, it would probably not be noticed unless someone demanded that it be noticed. (Different than if you were looking for it, and very unlikely to be noticed without prompting.)
Now, if you had some where on one the seats were in the car, and others where the sunroof was open and/or all the windows were open or if the seats were removed and sitting in the sun, then sure you'd have nice seats compared to trashed seats. But when all is said and done, I don't think you would have a problem using one in the other.
Cheers,
PH