I need set this parameter in command line when starting PLSQL Dev. (In some databases I forced to use single session mode by SESSION_BY_USER parameter.) Is any solution for this?
You could create a preference set for the databases where you want to use Single Session mode, and set the Session Mode preference correspondingly. See chapter 16.33 in the User's Guide for details.
From my experience, Session mode changes need a PLSQL Dev restart to be taken into account. So, I see no way of having it set depending on the database. Any suggestion?
You could use the PREFPATH command-line parameter to specify a directory for the preferences. One directory would contain the preferences including Songle Session Mode, and the other for Multi Session Mode.
This is a bit cumbersome though, so I have added a SesssionMode command-line parameter to the list of enhancement requests.