What I do is create two separate TSYS objects in the PSR500 - add the control channel frequencies for the Richland simulcast site into one TSYS and the Lexington simulcast site in the other. Add talkgroups from Richland Co AND Lexington Co into BOTH TSYS objects. And, of course, add other agencies talkgroups (SCHP, state agencies, helicopters, other counties, etc) to each TSYS as well. IE, you'll duplicate the same talkgroup list in both TSYS objects.
Assign talkgroups from both systems into your scanlists: for the Columbia FD scanlist, you'll have talkgroups from both the Richland and Lexington TSYS objects. In the Lex Co FD scanlist, you'll have LexCo FD talkgroups from both TSYS objects in it, etc. This allows you to continue to use the same scanlist arrangement, and you deactivate entire agencies (talkgroups from both TSYS objects) with one stroke when you disable a scanlist.
With both TSYS objects enabled (not locked out), you'll hear traffic from both the Richland site and the Lexington site. If you've appropriately tagged the TSYS object with something like "Palm 800 Lex" and "Palm 800 Rich" then you'll see that tag display alternately with the talkgroup tag in the display when the talkgroup is active.
If you prefer to only receive traffic from one TSYS, navigate to the TSYS object - easiest way to do this is to navigate to a talkgroup object in that TSYS and press the F2/TSYS button - then press Func+L/O to lock the TSYS object. Now, you'll only hear talkgroups carried by the other unlocked TSYS object.
If you want to use a wildcard to search for unidentified talkgroups in each TSYS, you'll want to program adjacent counties into each TSYS object. Like Fairfield Co into your Richland TSYS, and Aiken and Orangeburg into the Lexington TSYS. You can add those to scanlists, or disassociate them from any scanlist (essentially putting them into the NS (not scanned) scan list). Then your wildcard won't hit on these talkgroups.
Alternatively, you can program frequencies from both sites into a single TSYS, and set the Multi-site mode to STAT. This will force the radio to check each active CC it finds in the TSYS object. Checking the "Check all CCs" option will force the radio to check each active CC on each scanning "pass" through the object, so it will scan both Richland and Lexington sites on each scan cycle.
There are several downsides to this method. First, it slows the effective scan speed for any other objects you might be scanning (like conventional objects), but it's no worse than having both TSYSs enabled as mentioned above. Next, with this method, you can't easily deactivate talkgroups from one site or the other like you can by locking out an entire TSYS object. Thridly, you don't know which site an unidentified TG was carried on when it is found while using the Wildcard.
Hope that helps.