If you are in prison and are sentenced in another case, both sentences running concurrent from the same date. Will both sentences end at the same time

The sentences will not necessarily end at the same time even if they are running concurrent and given credit from time served from the same date that your brother took the plea in the original case.  Your brother, prior to being sentenced on the original case, may have had credit for time that he was in jail on the original case prior to entering the plea. If he was not in jail during the very same time on the new case, then the credit for time served would be different in the two cases.  Also, your brother upon being sentenced in the original case may have started getting some form of gain time on the original sentence.  It is unlikely that he will be given credit for that gain time on the new sentence even if it is concurrent.  It "may" be possible to negotiate some plea bargain that specifies the sentence on the new case will be co-terminous with the original case.  However, the wording for such a negotiated plea must be very specific and without question as to its meaning.  It may be hard to work out a plea bargain that accomplishes that.