When the TLServer starts it immediately tries to open up the TCP/IP port number that is stored in the configuration file. So when you try to open a second instance of TLServer the program will crash because the TCP/IP port is already opened.
There is a way of getting around it if you really want. You can install two copies of TLserver at two different folders. Each TLServer is configured with a different port number. You will then be able to start two instances of TLServer, one from each folder.
However, in order to connect to two different PLCs your PC will need two working serial ports. Otherwise one instance of the TLServer must close its serial port so that the other instance of TLServer can use the same serial port.
So It really isn't convenient to have two copies of TLServer running with two serial port and two TRiLOGI program just to monitor two PLCs.
The better way is always to use an Auto485 and connect to the RS485 port of two or more PLCs so that you can use a single TLServer to program/monitor multiple PLCs.