Of American-English origin, the colloquial phrase that makes two of us is used conversationally to declare, often ironically, that one shares the opinion, sentiment, predicament, etc., of the previous speaker.

This phrase occurs, for example, in the review of The Maidens, a novel by Alex Michaelides—review by Maureen Corrigan, published in The Washington Post (Washington, District of Columbia, USA) of Sunday 13^th June 2021:

Here’s Mariana sparring with Professor Fosca during dinner in his private quarters:
“He kept staring. His gaze was heavy, intense, lingering. She felt like a rabbit in headlights . . . .
‘You’re a beautiful woman,’ she heard him say, ‘but you have more than beauty. You have a certain quality — a stillness. Like the stillness in the depths of the ocean, far beneath the waves, where nothing moves. Very still . . . and very sad.’
Mariana didn’t say anything. She didn’t like where this was going — . . .”
That makes two of us, Mariana. As a Gothic seducer, the professor relies on lines more full of baloney than the Cold Cut Combo at Subway.

Implied in the phrase that makes two of us is what the previous speaker has just said—in this regard, cf. for example “That makes two of us that know it” in quotation 3 below, in which Jim Nasium recalled a conversation he had with Harry Barton.

The earliest occurrences of the phrase that makes two of us that I have found are as follows, in chronological order:

1-: From News of the Town, published in The Arizona Republican (Phoenix, Arizona, USA) of Wednesday 31^st October 1900:

OF ONE MIND.—The power of the press is illustrated by the fact that after reading in Monday’s Republican the report of Dr. Halsey’s sermon a Phoenix lady said to her husband: “I am ready to give a dollar a month toward sustaining a Young Men’s Christian Association.” “That lets me out,” said the husband, “I was thinking of doing that myself.” “No,” said the good wife, “you must give your dollar a month, too.” “Yes, that makes two of us.”

2-: From The Joliet Daily News (Joliet, Illinois, USA) of Thursday 4^th November 1909:

KENDALL EDITORS.

Another great editor from Kendall county, has been heard from. That makes two of us. John S. McLain, for more than twenty years editor of the Minneapolis Journal, one of the brightest dailies in the country, is taking life a little easier, having sold his interest in the Journal, and is now editor for our Joliet friend, George Thompson, on the St. Paul Dispatch.

3-: From Harry Barton Is Working Hard to Give Reading Fans a Winner This Year in the Tri-State League, by the U.S. cartoonist and journalist Jim Nasium (Edgar Forrest Wolfe – 1874-1958), published in The Philadelphia Inquirer (Philadelphia, Pennsylvania, USA) of Sunday 7^th August 1910:

READING, Pa., Aug. 6.
THE manager of a minor league baseball team has no sinecure. I don’t know what sinecure means, but I do know what it doesn’t mean. It doesn’t mean being made manager of a minor league baseball club. I found this out while talking with Harry Barton, manager of the Reading team in the Tri-State League. Harry knows it, too. That makes two of us that know it. Harry found it out in the school of experience. I found it out from Harry.

4-: From The Farmer’s Talk to Farmers, published in the Norwich Bulletin (Norwich, Connecticut, USA) of Saturday 20^th August 1910:

“You’re a farmer yourself, and a pretty good one as they run: you certainly don’t mean to label yourself a numbskull. That makes two of us, right here in this barnyard, that have admittedly got some sense and some reason and are more or less amenable to argument.”

5-: From the diary of ‘Grand-dad Woods’, published in The Tribune (Blair, Nebraska, USA) of Wednesday 21^st December 1910:

17—Every little while it’s Grand-dad don’t publish this and don’t publish that, and now as a bonafide city editor has his shop at the postoffice and that makes two of us taking notes, we suggest that the council turn the empty city jail into a hospital for nervous folks.

6-: From one of the unconnected paragraphs making up the column On Second Thought, published in The Topeka Daily Capital (Topeka, Kansas, USA) of Saturday 25^th February 1911:

“The name of the woman,” says the Associated Press in sending out the story of the Smith court martial, “is not known.” That seems to strip the case of about all of its interest.
A correspondent writes in to say this department exactly suits him. That makes two of us.
This department has its faults. But it can not be said of it that it ever piffled soft soap to the hysterical.

7-: From one of the unconnected paragraphs making up column 1, page 4, of the Wellington Daily News (Wellington, Kansas, USA) of Friday 10^th March 1911:

A nice line of Misses and children’s hats, trimmed and untrimmed and the prices are right. Mrs. Brunson. tf
One man is for me for mayor, agreeing to stand squarely on the dog platform. That makes two of us.
Miss Mabel Edmondson left this morning for Iola, Kans., to visit the family of D. W. Hicks.

8-: From one of the unconnected paragraphs making up the column The Scrap-Pile, by ‘El Capitan’, published in The Barnard Bee (Barnard, Kansas, USA) of Thursday 23^rd March 1911:

This eminent London doctor, Sir Algrath Wright, F. R. S., who denies that bathing is conducive to good health, is a man after my own heart.
So you think this stuff is pretty bum? Well, that makes two of us.

9-: From one of the unconnected paragraphs making up the column On Second Thought, published in The Topeka Daily Capital (Topeka, Kansas, USA) of Friday 6^th October 1911:

Old Ed Howe says he doesn’t find much pleasure in reading Emerson. That makes two of us.