This is something that can be accomplished but not via actual story objectives.
There is not story branching at all in that sense.
However you can use triggers and map dialogs to acheive this outcome.
What you need to do is to trigger a map-type dialogue to appear to replace the Talk with NPC objective that you currently have. In this map dialog branch the discussion out and classify each branch into the mob that you want to spawn or not spawn.
For every branch that you want to spawn a mob on (they don't have to be the same) add a new NPC group that is triggered to appear 'On Dialog Prompt complete' set to the last prompt on each of the branches (so the enemies don't attack until you finish talking.
However to really make it work you have to somehow prevent the player from moving on in the story without being able to ignore the talk to NPC.
There are two ways to do this... put up a forcefield or blocked door that won't come down until the map dialog is completed (though this may not be appropriate for all missions - you may also try using an invisible wall) or perhaps the best way is to make the object you use to trigger the map dialog an objective interact.
If you have any questions about how to actually do this... come and visit us on IRC server irc.geekshed.net, channel #starbaseugc. or via the chat page on StarbaseUGC.com.
Parts 4 through 7 of my triggers tutorials should teach you what you need to know to do something like this. In the end, you'll have to have a shared common objective regardless of the multiple pathing.