Check Force, Alexander Chane and Vladimir Gregoryevich Karlov, are freelance agents.
These two men are dedicated to fighting a powerful international enemy. Each man has, in his own way, learned of the existence of the group known as Force III. That very covert organization was born out of the ashes of WWII and is aimed totally at seeing the super powers head to all-out war. Agents have infiltrated the American CIA, Soviet KGB, and Chinese L-5, among other places, to keep the pot boiling in furtherance of that ultimate goal of world dominance.
Chane, formerly of the CIA, resigned from his Agency when his suspicions about a fellow agent, guilty of murder, are ignored by a superior who was clandestinely working with the murderer. Soon hitmen sent by Force III try to take him out and, when they failed, helped plant evidence against Chane. To escape capture, he left his country to hide out in the Dominican Republic.
Karlov had been the best the KGB had to offer but, though he loved his country greatly, had grown to despise the brutality and futility of its government. This drove him to finally defect to Britain. After considerable time held by the British government, unwilling to betray his homeland, he decided to go out on his own. Escaping several assassination attempts, he fled to the Dominican Republic.
The accidental meeting of the two men leads eventually to a loose partnership to combat Force III. Though naturally wary of each other after a career of not trusting the other side, they eventually come to realize that their common enemy will inevitably win unless they join forces. Even together their chances are slim to none but these two fighters know that they have to try, if for no other reason than Force III will not rest until they are dead. Besides the kill or be killed reason, these men are flat out ticked that their lives were ruined and they would like to return the favor.
Note that some lists of the books in this series also include the title Nightmare Island. I could find no such book by Ralph Hayes but I do have a copy of a book of this title by R.E. Hayes. It is a routine gothic novel of a woman in peril in a cliffside mansion on a storm ravaged island. No mention of Chane or Karlov.