Not in an easy way yet. Since all tab are independent instances of an extension/plugin. The one that get the startup focus during startup is the one that get created last.
Until that can be done, there is a workaround for it using script.
You can create a User Defined Command that is automatically run after startup that changes the tab focus.
To auto run a command after startup you need to add the -AutoRun=<command to run> parameter to MultiCommander.exe
MultiCommander.exe -AutoRun=AutoRun
And then create a User Defined Command of "CustomCommand" type, That you name "AutoRun" (or what ever name you choose for the -AutoRun= parameter), Then in that command you can write
MC.SetActivePanel PANEL=LEFT
