I don’t even recall seeing a master bath with 2 sinks. However, my dad’s previous house had 2 sinks in the regular main bath. I was only in the master bath once or twice but I don’t remember it being big enough for 2 sinks.
House built in 2004, 2 sinks in master bath. We requested that BTW. That way we each have space for our own stuff, and cuts down on the hair/toothpaste/morning mucus/etc. in the sink arguments.
House built in (possibly) 1810. Retrofitted bathroom from the 70’s. Two sinks, neither of which faces a mirror (?!).
I think if you have a house without multiple upstairs baths, then sharing becomes more likely and you need the extra sink. It’s also occasionally useful if you have to soak something.
Our house was built in the 20s, so only one sink, but two sinks is very common. When I hear the term “master bathroom” I picture two sinks side by side.
For houses that are big enough to have a true master suite I would say that two sinks are standard.
Our house was built in 1995, and has a good-sized master suite. But only one sink.
Have not designed a house or condo or apartment in the last 10 years that didn’t have 2 sinks in the master bedroom. I would say the market now is 2 sinks for sure. An older home may only have 1 sink but I would imagine most dwellings in the last 10 years you will find 2 sinks – especially if there is room for at least a 5’ vanity which allows 2 sinks.
Every house I’ve owned for the last 20 years has had 2 sinks in the master bathroom. I would say it is less of a function of how old the house is, but more of a function of how large your house is (i.e. what price range your house is in). It is a **very common **feature in more upper middle class homes today.
ETA: as far as the purpose, it is so that both members of the couple sharing the master can get ready in the morning without having to wait on the other person, i.e. convenience.
No, not a J&J. It’s an unusual setup.
Picture looking down on a shoebox from the top, long side running horizontally. On the left one third is the kitchen. Then going right is a door from the kitchen to the first closet. If you walk straight through you get to the bedroom, which is the last third of the shoebox. Mid way in the first closet is the bathroom, going up. The bathroom is about 8 feet with shower on the left, toilet ahead, and full length vanity on the right, with just the one sink. (There is also another door from the main part of the house directly into the master bedroom.)
At the end of the BR, in what would be the lower right corner of the shoebox, is another closet going “down” out of the shoebox shape (into the rest of the house). It’s smaller than the other walk in closet, about 7 feet by 7, and passing through it gets to another full bathroom, about 7 x 4, with conventional tub, toilet and three foot vanity (with one sink). This is the dead end bathroom. No other way to get in except through the closet.
The first bathroom is the nicer of the two. It has a cool sunken custom shower/tub, all done with tile. The second one has all the charm of an apartment bathroom.
Realistically, both seem to work only as master baths. I think the designer intended his and hers master baths (I’m assuming the lesser of the two is the “his”. 
) But with them being so far apart, and so isolated it’s just weird.
When I had cats, I put the litter box in the “his” bathroom. They liked that, isolated but accessible. Even though I no longer have cats, to this day it’s known as the “cat’s bathroom”.
Another vote that 2-sink master baths are all but universal in any US single family houses much after the 1970s. Maybe not in the very low-end “starter homes”.
They’re also all but universal in all but the smallest of condos.
We are renovating our master and chose to only have one sink. Two sinks seem to be a waste of counter space. We both shave in the shower, and when brushing your teeth you really only need to at the sink for probably 10 seconds.
We have two. Brand new manufactured home.
One bedroom apartment, so one bathroom sink. But my parents’ house was built in 1961 and has two bathrooms. BOTH have two sinks. And “Concealed Lavatories” revolving toothbrush holders. Tres chic.
Two sinks in Master and two sinks in the other full bathroom.
Condo; Built: 2012
That’s what we have. Our subdivision was built in the late 80’s and all the houses have two sinks. I know a few people with only one sink in their master bath, but the vast majority have two.
We have two. The house was built in 1953 but the bedroom and bath were added sometime in the last 10 years by the previous owners.
I was watching a House Hunters show over Thanksgiving and one of the shows had the woman absolutely demanding a double sink. So, not uncommon any more.
It used to be an upper middle class thing.
Now it is a McMansion thing.
So you are less likely to see it the further down the scale of houses you go. In my area newer than 1990 and with a master bath - 2 sinks.
Older than that and it is hit or miss depending on what part of town the older house is in. Downtown - very likely, skirts of town - maybe, anything approaching rural - one sink.
I only have one in the master bath, but I will say that during my house hunt I ran into a lot of flipped houses, and two things they seemed pretty consistent on were granite countertops in the kitchen and double sinks in the master bath.
Neither of which I wanted.
I just don’t see the practicality here. I would like my own sink, but I would like my own whole bathroom, too. (My house has three bathrooms, so that is doable.) That is to say, if it’s next to somebody else’s sink I might as well be back in the dorm.
As I stated upthread, both houses I have owned, and every colonial model my wife and I have ever visited, had two sinks in the master bath. Some had a single vanity and some had dual vanities, but always two sinks.
In fact, when we were having our latest house built, the default for the guest (upstairs hallway) bath was two sinks in a single vanity. Because of the necessity to change the vanity and the plumbing, a single sink was a special order which, thankfully, we opted for. We love it because the guest bath’s vanity has a large 60" by 24" surface, which is great for decorating.
Below are links to two photo’s of our guest bath vanity/sink combo:
Our home has three bathrooms. The master bath is my gf’s bathroom and has two sinks. One is where she washes her hands/brushes her teeth. The other has some details for makeup application.
My bathroom (the slave quarters) has one sink, as does the guest bathroom.