"separate agreement" is correct and usable in written English.
An example of using "separate agreement" might be: "The two parties involved in the negotiation agreed to sign a separate agreement."
"It is a separate agreement," he said.
"I never signed a separate agreement to the rematch.
(Slot machines are covered under a separate agreement).
This is part of a separate agreement Iran has made with the International Atomic Energy Agency.
ICE also had a separate agreement to use bed space at the jail.
Mirage and Boyd will continue to work together under a separate agreement reached in July.
First, there's the individuality of each author's agreement — a separate agreement with each publisher for each publication.
Without an extension of the resolution or a separate agreement with the Iraqis, American troops would have to cease operations.
In a separate agreement, it also gave Johnson & Johnson the rights to sell the drug for all uses overseas.
BP also retains hopes of clinching a separate agreement to develop non-associated or "deep gas" at the ACG fields.
A separate agreement calls for Liberty to receive about $25 million if The Star Bulletin is sold.
