During the campaign, you can add the amount for whatever add-on you want to your pledge (e.g. for a second copy of the game). At the end of the campaign, you will get a survey. There you can select which add-ons you want and pledged for. All money pledged during this Kickstarter will count toward the stretch goals.
If you prefer, you can also order add-ons during the survey after the project closes, but doing so doesn’t move us closer to our stretch goals.