It is not clear what the original poster's issue was - it was very likely due to not using separate ini files or not using the "portable" option in the installer.
Installing two copies of mIRC is fine, many people do that. However I was not able to reproduce your issue. I just tested by installing two separate copies of mIRC, one as a normal install and the second as a portable install. Their settings did not conflict. The normal install continued to use the AppData location and the portable install only modified and used the install location. Are you sure that installing the portable option is the cause of this issue? Are you able to reproduce it?
