This isn't trigger. It's just force settings... Unless you're talking about the multiboard that shows both teams.
You set the different teams in the Force tab of Scenario Properties window.
Go to "Scenario" at the top of the WE screen and select "Force Properties", this way you'll be directed to the Forces tab.
In the tab, you will check the "Use Custom Forces" and "Fixed Player Settings" boxes
Then click on the "Add Force" button to create a second one.
Finally, you'll drag the desired players to each force.
Don't forget to check the "Allied" box in both Forces, to set all the players in that force as allies.