I don't monitor them myself, but I would say "no" they don't carry the exact same traffic. You'd have to monitor both and see if you're missing anything. Most "common" talkgroups in your area probably are carried on both most of the time.
I would bet that Harbison carries more Lexington Co talkgroups than Richland would. But Harbison might not carry all of the same traffic that Richland Co simulcast would. Richland might carry some Kershaw Co or Orangeburg Co talkgroups that Harbison might not carry. Harbison might carry more SCHP Troop 2 talkgroups that Richland doesn't typically carry. And what is heard on each site might vary, and you might be able to determine a pattern in terms of time of day.
I don't recall which radio you use, but you can probably get Trunk88 or Unitrunker running on your PC and let that software monitor each site for a period of time, at least a day on each if not longer. Then compare the lists of talkgroups to see which talkgroups are carried on each.
