I'd imagine the idea is to avoid d SHT2A and SHT9A being on the RT at the same time, infact Eurocontrol has a program designed for that very purpose.
Over the last 20 years though many destinations have received similar callsign groups, such as FRA outbounds having a N and their returns a U- BAW902N/BAW903U.
The SHT callsigns are indeed operated by the same crew as all are on the BA AOC (BAW, SHT and XMS)