Hello there.
According to this post, which is as recent as I could find. Combining multiple streams into a single stream with Wowza is:
a workflow that we are aware of and are looking into how we can support it in future versions of Wowza Server.
-Lisa
I am not aware of this feature being supported at this time.
You might however look into VLC mosaic for a workaround.
Alternatively, you could post a request to the find a consultant forum and see if any of the developers there have a module or workflow you could purchase. Or, if anyone could develop a module, or help you get this working.
Kind regards,
Salvadore