First of all, each PLC must be set with a unique ID. You can do this from within TLServer Serial Port Setup screen. You will have to do this one at a time using the RS232 port.
Once each PLC has its own unique ID, then when they are linked together to the PC with the USB-RS485 converter, you should try to test each PLC ID from within TLServer first.
E.g. if you have three PLCs: 01, 02,03
Try to send the following command one at a time:
@01IR00*
@02IR00*
@03IR00*
If you get a response from each PLC then your setup is working fine. When you try to use TRiLOGI to address to each PLC, simply enter the ID of each PLC at the login screen and not use the "Detect ID" button. You will then be able to switch from one PLC to another from the same TRiLOGI program.