beta.vMixcall.com seems to work really well.
Not sure why it's still in beta.
The ability alone to switch devices is killing the "old" version where users never seem able to figure out how to switch devices.

The desktop share function also works really well.
The only downside is that it is left to the user to toggle between camera and desktop.

Few users are able to do anything but selecting their presentation in the first place.
After they have turned on the desktop sharing - they rarely figure out to switch back or even back and forth between camera and desktop.

Why not simply send two streams to vMix?
Not only will it allow the producer to switch back and forth on behalf of the guest, but also allow the producer to do Picture-In-Picture (PiP) etc.
If bandwidth is a concern, an option for producer could be to activate "Low Frame Rate" for the desktop sharing, thus conserving bandwidth as presentations rarely contains animations/video content anyway.

Best
Jon

On conference, audience need to see slideshow and the presenter at the same time (on window layout).

Personally I use a OBS Ninja modification to share screen and I created a platform that remote presenters use to share screen and join to conference when vMix ID activated (using iframe). I ask manually a new vMixCall on presentation time and automatically (with my platform) I send it to presenter dashboard to join and activate camera (desktop share is already activated because I have unlimited ids;) ).

But it would be simpler if vMix Call send the desktop and camera at the same time, and you need to be a coder to make this system.

+1