We should find the guy that hooked that up and ask him what he was thinking. Oh wait, it was me.
I have spoken about this at length in other threads, but the long and short of it is that there is nothing wrong with multiple appliances per flue, the issue is when one of them is a "solid fuel appliance". When you hook a wood burner into the same flue as an oil or gas unit, and there is a chimney fire, the fire can draw air through the other appliance and its draft control. Anthracite does not create creosote or cause chimney fires, and actually requires a draft control of its own...but the code does not distinguish between wood and anthracite. If it were two oil boilers, the concern would be the size of the piping and flue. This particular chimney handles both boilers firing without issue, but since the video was filmed 3 years ago I have changed the flue piping arrangement and controls so that only one boiler can be used at a time.