I have been able to have multiple streams on iOS, but not easily. GarageBand might be able to do this via imports, but I’m not sure it it lets you queue/play multiple samples simultaneously like it will on macOS iirc.

If you are playing music through Spotify in the background, foreground audio in Snapchat still plays normally while recording and playing back just recorded snaps, as well as snaps or memories you have prepared earlier. Sometimes you need to start playback on Spotify again via Control Center, because Snapchat steals focus or takes priority for audio output or something, but it is just part of the jankyness of this workflow, which is probably not intentionally designed to be used the way I use it. If you combine these quirks with Screen Recording, you can make simple audio loops by recording simultaneously via Snapchat and iOS Screen Recording, then use those videos as uploads to Snapchat to stack the loops over each over by selectively queuing them, with audio from a video in Snapchat playing at the same time as audio from Spotify.

It’s kind of a weird workflow, but it’s neat that it works. It feels intentional, as most apps stop background audio playback when starting recording on the same device, but at least Snapchat does not do this, so it’s at least technically possible.