If you enable the 'IA engine', then it should allow multiple hosts on port 502. However, you might need to reduce the various timeouts so the TS_MEI 'hangs up' and idle socket faster.
What I've seen, is the hosts start 'thrashing' sockets, so the TS_MEI eventually runs out of free sockets, with all sockets 'waiting' for more requests.
The other thing which happens, is some slaves disable their serial port after seeing a message, so you need to add a delay between requests (aka: 2nd master cannot poll slave too rapidly after 1st master). I don't remember the TS_MEI config so well, but there should be an RTS turn-around/toggle delay.