I don’t think the official Reddit app has ever supported this, unless it’s a new feature?
Only 3rd party apps supported this, but a lot of those have been killed off. You might still be able to access this feature through old.Reddit:
https://www.reveddit.com/v/NoStupidQuestions/comments/9t50jp/what_is_a_multireddit_how_do_i_access/
I’m not sure if you meant “multi-communities”.
In the fediverse, PieFed is a useful one where you can combine different communities into “feeds”.
For instance, there are a bunch of different “technology” communities, but all of their posts are combined into this feed: https://piefed.social/f/tech
In android, if you go to the website, then click on the 3 dots -> Add to Homescreen -> Install
Then you’ll be able to use PieFed and browse communities in the way (i think) you’re asking.
Otherwise this is also possible with the Android app “Eternity” which let’s you create your own “Multicommunity”. The advantage to the PieFed one is that someone else has already scoured the fediverse for you and combined them.
I’ve seen some streamers set up so that they can stream on multiple services simultaneously. It might be something worth looking into.