"united strength" is correct and usable in written English.
It is typically used as an expression of solidarity or group strength - for example, "We must stand together with united strength to achieve our goals."
On the Continent, Austria, France, and Russia could never bring their united strength to bear effectively on Prussia.
She is also heir to all our united strength and loyalty … That it should be a golden age of art and letters we can only hope but it is certain that if a true and lasting peace can be achieved … an immense and undreamed of prosperity, with culture and leisure even more widely spread can come … to the masses of the people".
"A civil constitution implies in it united strength, various subordinates under one direction … a superior faculty whose office it is to adjust, manage, and preside over them," (Sermon III §2).
With this object he had brought together into the city a great multitude from all parts of Italy. 3 But he was driven from the city by the united strength of his college and the optimates, and set out for Campania.
We have to summon our united strength, our full intelligence, our own super power.
"You tear apart the baby's rattle and see what makes the noise inside, but there is a veil covering the unseen world which not the strongest man, nor even the united strength of all the strongest men that ever lived could tear apart.
She chose love and takes joy in making her dancing show it while seeking to unite strength with elegance.
It is a beautiful sight to see, us all united in strength rather than shattered and scattered in defeat.
-- Members of an Irvine synagogue are united "in strength" after their building was defaced with anti-Semitic graffiti.
You will hear the sounds of brothers bonded together by ooze and united in strength under the rallying cry of YSL.
The sub-committees would keep in close contact with the central committee because "united our strength and resources are more than adequate to our defense in any possible event.
