Once you've created the first target on your campaign dashboard, you'll be able to click the orange +new campaign button.
Click the +new campaign button, and repeat the process for adding a target to the ad. Select between interest groups and subreddits, then adjust your timing and budget before clicking "create".
Repeat this process for however many targets you'd like.
There are no restrictions from one campaign to the next, and they can overlap and be a mix of subreddits, interest groups, frontpage, and location. Your ad's targeting plan is truly customizable. The copy and thumbnail will remain the same in each target, and the vote count and comment section will be centralized in one ad.