The phrase "acquaintance of both" is correct and usable in written English.
It can be used when referring to a person who is familiar with two individuals or groups.
Example: "She is an acquaintance of both John and Sarah, having met them at various social events."
Alternatives: "mutual acquaintance" or "common acquaintance".
An acquaintance of both victims said they were homeless.
"Liberman took Si out to good restaurants and taught him which fork to pick up," recalls an acquaintance of both men.
A Harvard acquaintance of both Dean's and Loeb's remembers the two as "making quite a study in contrasts: Dorothy was small and dark; Arthur was tall and pale.
Professor McIntyre liked to engage fellow patrons on the subject of race, according to one regular customer, Daniel Morgan, who considers himself a close acquaintance of both Professor McIntyre and Ms. Davis.
Officials said Mr. Robinson, of 772 Henry Street, in Red Hook, was an acquaintance of both Al-Taya's and a former boyfriend of hers who had been arrested at least twice on claims he had abused her.
In an interview recorded by an investigator, one acquaintance of both men said the former partner told him about Mr. Chiacchia's positive status on Easter, months earlier than he told the Sheriff's Office he had made the discovery.
In Albany, relatives, friends and acquaintances of both men insisted they were innocent.
Mr. Mack, who disputes suggestions by acquaintances of both men that he and Mr. Peek had clashed, says that "CIT was a bigger opportunity" for Mr. Peek.
FBI agents have interviewed witnesses and acquaintances of both Levy and Condit from Virginia to California's Central Valley in recent weeks.
The household was augmented at various times by the composers William Walton and Constant Lambert, the artist Nina Hamnett, and sundry acquaintances of both sexes.
One of the most notable and respectable of the travelling lecturers was James Ferguson FRS, a Scottish astronomer and probable acquaintance of Joseph Wright (both were friends of John Whitehurst).
