This is a great question. The at.js library does not support multiple mbox responses with a single call. While the Target REST API does have a method for this. It is limited to form based activities in Target. If you are using the Target global mbox and the visual composer you can deliver multiple activities to a page with only a single call to Target.This is actually how at.js and the visual composed activities natively work.
Also worth noting, if you are using form composed activities because you prefer to write your own code, these use cases usually can also be handled in the visual composer by using the custom code option to insert your own code blocks to the page. So you could get multiple custom code offers delivered in a single call to Target.
Our next Adobe Target Skill Builder Webinar is right around the corner on May 5th, and will be presented by Adobe Target Product Management on migrating Adobe Target’s mbox.js to At.js. Check out this community discussion to register today. Hope you can make it!