The phrase "a formidable duo" is correct and usable in written English.
It can be used to describe two people or entities that work well together and are impressive or powerful in some way.
Example: "The two scientists formed a formidable duo, making groundbreaking discoveries in their field."
Alternatives: "an impressive pair" or "a powerful team".
The Ducks boast a formidable duo up front in the form of Ryan Getzlaf and Corey Perry, along with the Ageless Wonder, Teemu Selanne.
They were a formidable duo... Hogwood cut an unmistakeable figure with his trademark beard and sandals and seated at his harpsichord.
At last summer's World Cup he formed a formidable duo with Borussia Dortmund defender Sokratis Papastathopoulos and helped Greece reach the Last 16 where they were eliminated by Costa Rica.
In a perfect world, Francis Coquelin will become the world's greatest holding midfielder, Aaron Ramsey and Jack Wilshere will form a formidable duo and Olivier Giroud will leave Diego Costa and Sergio Agüero in the shade.
Nigel Howarth, the legend of the Ribble Valley, king of the Lancashire hotpot, rules in the kitchen, although the spritely Lisa Allen drives the team along, and they make a formidable duo.
The Newest Members Are Not to Be Trifled With The court's most recent additions, the Obama appointees Sonia Sotomayor (two years on the bench) and Elena Kagan (one year), have quickly become a formidable duo on the court's left flank, with the promise to serve as a 21st-century version of Thurgood Marshall and William Brennan.
There hasn't been a sadder or less formidable duo since the Wonder Twins on the old Super Friends cartoon – two kids from another planet whose superpowers involved one turning into forms of water and the other turning into shapes of animals.
The formidable duo of Oliver Goldstein, an LA indie-dance veteran and sought after songwriter, and Vaughn "U-Tern" Oliver, a seasoned Vancouverite scratch DJ, has been lending skills to various artists since the turn of the decade, but 2014 has found them stepping into their own as an impeccable catalogue of releases lengthens and the profile of their DJ sets ekes higher up festival line-ups.
The addition of Gomes gives the Nationals a formidable catching duo, as the 31-year-old joins newly acquired Kurt Suzuki.
