Like Jeff said.
TBC can't modify rinex files it just reads them in.
In TBC if the rover files started and ended within a short period of time, TBC would concatenate the files and process them as a single vector when processed with a base station file. If the rinex files overlaps they would process as two trajectory's.
The primary software to modify rinex is the command line software TEQC, maintained by UNAVCO. You can find the latest and last version here TEQC | Software | UNAVCO
TEQC can combine rinex files among many other things but it is a command line software so it's not very easy to use unless your a frequent user of CMD.
WinTEQC is a user friendly front-end for the TEQC software.
it is found here,
However it failed run on my Win10 laptop so read the TEQC help. If someone has advice running WinTEQC please comment.
fyi, maybe I'm the last to know