Inspected this home yesterday - the panel has 2 20-amp breakers which have a single label “Disposal/dishwasher”. One controls the disposal, the other controls the dishwasher.
But looking at the photo, why would it be installed this way?
This is a subpanel and I couldn’t trace the wires to their origin, but I see a couple of breakers with this done.
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What did the other end of that circuit “look” like? Marked “Disposal/dishwasher” doesn’t really tell us anything useful.
Ref: Wanted: Good MWBC (multi-wire branch circuit) Diagram - #50 by ruecker
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According to that diagram you included - a tie would be needed between the 2 breakers… Just spoke to an electrician friend of mine. Said this is common, but kind of “old school”, but that Handle Tie would be required. The 2 circuits apparently share a common Neutral.
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MWBC-multi wire branch circuit. Requires a handle tie or two pole CB. Those loops on the feeder are horrible. Also there are EGC’s on the neutral bus.
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It’s wired as a MWBC(multi-wire branch circuit) to save money. Two individual circuits for the two appliances would be 2x Line, Neutral, and Ground. That’s 6 wires. With a MWBC it is Line 1, Line 2, Neutral, Ground. That’s 4 wires. This type of wiring is allowed and is still used on dishwasher/disposal circuits on I would say 99% of new construction in my area. You should read up on the differences between this type of wiring and regular wiring for a circuit as well as other requirements such as handle ties.
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Like others have already explained, they could have ran a 3 wire to a dual receptacle under the kitchen sink. They would have broken the tab on the hot side to create two separate circuits, one switched for the disposal and one always hot for the dishwasher.
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Yeah - for some reason, I’ve never come across this. I know now! And you’re right - the graphic makes it crystal clear. And I had already noted the neutral buss. It’s a $2M house… seems silly to save money on a couple wires, but …
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Over time, bad habits remain constant.
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What bad habit? Where?
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The electrician wanted a few more bucks that night at the bar
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When you are going through thousands of feet of wire it adds up. Not to mention less staples and less labor.
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Why would you run two cables to one outlet? Richard posted the exact diagram (minus the disposal switch).
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Richard’s image has a few flaws. The note makes it sound like handle ties are only needed if the MWBC serves a single device. And the receptacle is shown upside down. 
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The multiple cables in the large KO are wrong also.
That makes no sense. The note is only referencing the illustration it is next to. The receptacle will work upside down or sideways, electricity won’t care. The OP was discussing a multi-wire to a single under sink outlet for the disposal and dishwasher, not multiple outlets as in Randy’s drawing.
Thus my question to the OP…
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I don’t see where the OP stated it was a single receptacle. He stated it was for a disposal and a dishwasher but didn’t say how many receptacles were involved. He also said other breakers were this way.
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Sorry - was looking through my pics and got sidetracked! The disposal is plugged in to the receptacle under the sink, but the dishwasher is not in plugged in there. The dishwasher power source is hidden.
So, what’s your point? It’s a multiwire circuit to a dishwasher and a disposer. You don’t like Richard’s graphic. Randy’s has four outlets. Let’s see, . . . what are the odds 4 outlets are not involved here?