Recording a two speaker event with large audience
If I was doing it I would probably give both speakers a wireless lavalier (lapel) mic, and record each to a separate channel. I would have two cameras (at least), one generally staying wide (which can be a static camera if you don't have more than one operator), and one getting close shots, and following on the action or dialogue, so when one of the speakers is speaking it focuses on them. 



You use the wide footage to cover the times when the close-up camera is switching back and forth. If there's a lot of to-and-fro dialogue between the two you could use a third camera, or frame the closer shot as a two-shot. If you can shoot in 4k and you're releasing in HD you can also shoot wider and use cropping to get close-ups.

You could also get audience reaction shots and as you said, the feed from any AV presentations should be recorded too. Keeping the cameras out of the way of the audience can be tricky, if you have long lenses that helps, because you can move them to the back, but it depends on the geometry of the auditorium and the gear you have available.

Lastly, the best way to edit the separate streams is to use the multicamera editing capabilities of your NLE. Premier, FCP, resolve all can do multicamera editing, and so do most of the other major NLEs. Using multicamera means that once you've synched the cameras (use the on-board sound from each camera to do this) you can watch them all playing simultaneously, and choose which angle to cut to with the keyboard, a bit like a live vision mixer. It means you can edit at close to real-time once you get the hang of it.