has any one ever heard of a bike having both a free cassette and a fixed gear cog on the other side of the hub? is that even possible without custom modifications.

So that would mean you'd have a rear derailleur to change gears on the cassette. This would make the fixed gear impossible to use with the longer chain and the derailleur in the way.

Yes, it's possible. ISO cogs use a 6-bolt pattern to attach to a hub. Most hubs meant for disc brakes will match the bolt pattern.

Only issue is that it will be 135mm OLD and not have much room to narrow that.

Or you could just put a freewheel on one side of a standard flip/flop hub.

But he specifically said a cassette, not a freewheel, although I cannot imagine why you'd want to do that.

It wouldn't be an issue on a mountain bike, since we're speculating wildly.

I saw an electric bike that was set up sort of like that. Cassette on right side, reverse threaded freewheel on the left with a chain going up to a motor on the rear rack. The battery was slung off one side like a pannier. Total crap design.

I don't think anyone makes exactly what you're saying, (except for the disc MTB hub.) But I think it would be a really nice feature. SS cassette cogs are cheaper and quicker to swap out than thread-on freewheels.

Chain tension would be a big issue here....