Adding an additional dishwasher
Well, there might be a couple of issues here. But I don't think it is all that uncommon to have two dishwashers.

Of course, the most important thing is going to be how you connect the drain hoses. If you have a garbage disposal with a drain connection for a dishwasher, it's only going to accommodate one dishwasher. So you could connect one to the disposal, and connect the other into the tailpiece under the sink basin opposite the disposal. You have to put a high under-sink loop in both drain hoses at the very least. Check your local code, though. There's a good chance you have to install an air-gap device on each drain hose, and you might have to mount the air gaps above the counter top.

So the issue you might be reading about is backflow, which is when dirty water flows backwards into your dishwasher. Generally, the high loop(s) and, if required, the air gap device(s) will prevent this.

Another related issue is that it's important to drain the dishwasher(s) into the sewer line before the P trap. The water plug sitting in the P trap (combined with proper venting) is what keeps sewer gases from pushing back up into your house. If you connect the dishwasher drain hose(s) downstream of the P trap, you subvert this mechanism and sewer gases will push up into your dishwasher.

Back pressure might be a different matter, if the drain pipe under your sink is too small to handle both dishwashers draining at the same time, or if the pipe is clogged, then the water pressure could be high enough to force some of the water back up the drain pipe into your sink.