When You launch the SDM URL "http://server:8080/CAisd/pdmweb.exe"-> it will direct you to either "http://<server>:8080/CAisd/pdmweb1.exe" OR "http://<server>:8080/CAisd/pdmweb2.exe" Based on the Load on each web-engine.
To check the load, run the "pdm_webstat" command. It will show cumulative sessions, most sessions at a time and current active sessions on each web-engine. Below is an image that shows an example with single Login on each web-engine.
If "cummulative Sessions so far" from the above command output is equal on all web-engines then 1st Login will direct to: pdmweb.exe 2nd Login will direct to: pdmweb1.exe 3rd Login will direct to: pdmweb2.exe
If "cummulative Sessions so far" -> Is not equal on each web-engine -> then all the login request will be directed to the web-engine which has the fewest logins until the sessions are equal on each web-engine.