They are MUCH nicer than the narrow dishwashers, too. We had one of the narrow ones in our old apartment, and routinely ran into problems fitting things. The drawer kind makes much better use of its space.
I don’t recall the water usage being all that much less than any modern, high-efficiency unit. Ditto for quietude.
Compared to your basic $400 Kenmore, any quality unit is going to have better power and water efficiency, more durability and quieter operation. The PK drawer models are very niche-y products, even compared to, say, Bosch.
The water savings comes from only having to run half the dishwasher. Again, it’s only a benefit for people who find themselves running half loads in a standard dishwasher, which could be true for a lot of older couples. The price is around $1200 retail, which is similar to a Bosch.
We bought a replacement refrigerator (they’re hard to find in basic white!) I wish I would have paid attention to a noise rating - the thing is loud!
Besides two dishwashers, I’d go go for two microwaves. I have a family member who is vegetarian and it’d be nice to cook his veggie burger while heating frozen vegetables at the same time. I’d use two microwaves simultaneously much more frequently than using two ovens.
I did a variant, I had an apartment that had a cook-n-clean - imagine a stove with integrated microwave over the counter, and an electric 4 burner range, but instead of the oven under the burners, it had a dishwasher. My kitchen was so damned small, I just kept everything in the dish washer so I could have cupboard space for food. I had the classic place settings for 4 in white corningware, flatware for 4, a 1 gallon stock pot, a 3 quart dutch oven, and one of the la creuset combo 2 qt sauce pans that used an 8 inch skillet as the lid, and assorted kitchen tools.
WE have 2 dishwashers and it is great. We run both almost every day. I’ve got 3 kids, and have taught them to put the dirty dishes in the dirty dishes dishwasher. We don’t have a sinkfull of dirty dishes. Honestly, the biggest issue is with unloading.
If I was a bachelor with 2 dishwashers, I’d rarely use the cupboards. 
One big disadvantage of this idea not mentioned so far is the much larger amount of room it would take.
Dishes in dishwashers are stored much more loosely than in cupboards (they have to be; they must be spaced apart do they can be washed clean). In cupboards, plates are stacked right on top of each other, bowls & cups are are nested together, etc., so you can store the most dishes in the smallest space. You couldn’t do that if they were stored in the typical dishwasher racks.
Unless, of course, if after washing, you moved the dishes around to stack them much tighter together. But if you’re going to do that, you might as well move them from the dishwasher to regular cupboards.
The kitchen may look fine when this is done. But do realise that those dishwashers will age, and that in 8 to 10 years one of them will fail. You will then replace it with that year’s model. Then another 3 years later, a second one will fail. You will replace it with *that *year’s model, which will be different from the originals and from the newish one. Within 20 years, your kitchen will look like a showroom.
It’s not that simple. Modern dishwashers, especially quality ones, use very little water compared to older models. (Just like modern front-loader clothes washers use as little as 1/10 the water of top-loaders, especially older ones.)
A random 24" Bosch spec chart shows 2.9 gallons per cycle, 624 gallons per year.
A F/P single-drawer shows… 2.0 gallons per cycle, no annual rating I could find.
Below the level of significance unless you’re living from an RV water tank or on a spacecraft. 
Yes, yes, it’s almost a gallon, and one-third more… but most people probably waste a gallon a day on trivialities like letting the faucet run.
One dish drawer for everyday dishes would make sense for both washing and storage. However if you entertain or want to wash the pots and pans in the machine you’ll still want a full size unit as well. I’d do it if I had the kitchen space.
I say go all the way with this idea. Install waterproof doors on each exit from the kitchen, shower heads on the walls and ceiling, and a drain in the floor. Make your whole kitchen into one big dishwasher.
By code, each individual kitchen appliance needs it’s own “home run” circuit.
That might be going a bit too far, although it would be nice to never have to clear the table. Now, a garbage disposal installed in your shower drain… that can be a real time saver!
So… it uses 1/3 less water and the two-drawer system costs about the same to purchase. It’s also one of the Top 5 quietest units on the market.
Pretty much everything I said to begin with.
But it’s two drawers! Did you know that, Mr. Smarty? 
Years ago, a friend and I dreamed up the idea of an all stainless steel bathroom with a floor drain and water jets installed, strategically on the walls and ceiling. A few times a month (more or less), you’d close the door and hit the self cleaning button and it would spray itself down.
At my college each wing of the dorm had a shared bathroom. The bathroom had a few stalls and 4 or 5 showers (divided). Each morning the janitor would come in, connect a hose to a wall mounted spigot and spray the entire place down with what was basically a pressure washer. Must have been a lot easier than scrubbing it on her hands and knees.
Versus a cabinet, which will continue to be usable indefinitely.
“This food was in the shower with you?”
That’s not a wacky idea at all. I was chatting with a neighbor a while back, and she mentioned that she had back problems and her dream would be to have that kind of thing further off the floor.
As a tall person, I concur. Cabinets where the bottom foot is for very rarely used things (the kind of thing that goes in back of the tallest upper-cabinet shelf). Counter height raised to partly compensate, I could easily work with counters 6 inches higher than standard.
Double ovens: Oh my yes. We have double wall ovens and the bottom one is used to store the roasting pans and cookie sheets, except on holidays when we wish we had triple ovens. Most double-oven sets have fewer features on the bottom one; the original ones in this house didn’t even have digital controls for the bottom one, let alone a self-cleaning cycle.
Back to the OP: Cute idea, but I think two dishwashers would come pretty close in actual usage. As others have said, you have one for dirty, one for clean. Just don’t mix them up ;). Then you need a cabinet to put the things you washed but aren’t going to use again for a while. I sure wish we had two dishwashers; right now we’re running the unit at least twice a day because we’ve got housemates, and that will get worse in about a month when we host a student for the spring.