The phrase "one of the only two" is grammatically correct and can be used in written English.
It is typically used to emphasize that there are only two options or possibilities. Example: She was one of the only two survivors of the wreck.
Pick one of the others and the waiter will still bring you one of the only two on offer, but you won't have chosen which.
The fact that she took a one-off walk-on job on "Transparent" and turned it into a recurring guest star role as one of the only two trans characters on a critically acclaimed, groundbreaking show is treated as secondary to her stripper life.
When her family moved there it was one of the only two houses on that side of the block; the other, No. 71, belongs to Morris Gest.
Erected in 1875, and now one of the only two banks on Broadway, it has a lofty skylit dome that presides over the transactions of Williamsburg's mix of Hasidic businessmen, Hells Angels, Puerto Ricans and artists.
A barrel bomb hit one of the only two remaining hospitals on Thursday, the Guardian reported.
Doctors Without Borders ―also known as MSF for its French name ― runs one of the only two medical clinics on the island, according to the report, after another one closed its operations in May.
In addition, it is one of the only six ports on the map marked with a red circle.
"One of my trainers is one of the only four Zone-certified specialists in Italy," Mr. Labadia boasted.
In addition, Aceh is also one of the only six ports marked with a red circle.
Multivariate analysis for the most influential death-related risk factors shows MV as one of the only three relevant variables.
Apparently this is one of the the only two in known existence.
