Hello Skyler,
MistServer doesn't do transcoding, which is what you would need for the task you're describing.
What it can do, is transform the multicast streams into formats compatible with for example OBS Studio (among others, our MP4 live output is compatible), which you can then use to create the "multi-view" and send that as a new stream elsewhere (or to the same server instance as a new stream name).
I should note a bit of ffmpeg scripting can already accomplish what you want as well, though it is harder to set up as ffmpeg has no graphical interface like OBS does.
Hope that helps point you in the right direction!
Regards,
Jaron Viëtor
CTO, DDVTech