It's difficult to explain, but there really weren't two different regiments. In many cases a single Confederate regiment or battalion operated as two different entities. If officers and men were unable to reach their command for one reason or another, they might be temporarily assigned to another unit. In this case there may have been enough officers and men in the TM theater to serve together under the original command designation, making it appear as if there were two different 17th Texas Cavalry Regiments.
I reality both groups on different sides of the Mississippi represented the same regiment.