『Longma Dictionary of Contemporary English』 で名詞の police をひくと：
po‧lice 1 S1 W1 / pəlis / noun [ plural ]
1 the people who work for an official organization whose job is to catch criminals and make sure that people obey the law :
Police surrounded the courthouse.
2 the police the official organization whose job is to catch criminals and make sure that people obey the law :
Quick! Call the police!
By the time the police arrived the man had fled.
He was arrested by the police for dangerous driving.
He plans to join the police when he leaves school.
→ military police , secret police
Police and the police are plural :
Police are still searching for the murder weapon.
The police were called.
Do not say 'a police'. Say a police officer , a policeman , or a policewoman .
最後の行の Do not say 'a police'. Say a police officer , a policeman , or a policewoman という用法に従えば２人の警官は two police ではなく two police officers のようにしないと文法上正しいとはいえないことになる。
そして、２つの警察（組織）であれば、これまた two police ではなく、two police forces となる。
この文法に関しては http://www.place-inc.net/column/columnframe.html に詳細な説明がある。
ただし、two police という表現も普通に使われている：
たとえば、ある会社（遊戯施設）の盗難事件の現場に３人の警官がやってきて取調べる場面で ― Mr Parry makes a telephone call. Very soon four police arrive in a big white police car. They ask a lot of questions.・・・Now the police want to talk to the workers. One stay with the visitors and three police speak to the workers・・・（『The Fifteenth Character (Oxford Bookworms Library ― starter level 』 より１部省略して引用。 police という単語全てに注目して私が太文字にした）
警官の類語 （『Longma Dictionary of Contemporary English』 より） ― 太字はk.y.
police officer ( also officer )
a member of the police. In British English, police officer is used especially in more formal contexts, for example in news reports. In everyday English, British people still usually say policeman or policewoman : a senior police officer | He was sentenced to life in prison for killing a police officer. | He is the officer in charge of the case. | Officer Fayard (= in the US ‘Officer’ is used in the title of police officers )
a man who is a member of the police : an off-duty policeman | He’s a former policeman.
a woman who is a member of the police : The girl, accompanied by a policewoman and two social workers, was seen in private by Sheriff George Crozier.
used in the job titles of British police offiicers. PC means ‘Police Constable’ and WPC means 'Woman Police Constable' : PC Keith Fletcher | WPC Susan Larkin
a police officer whose job is to discover who is responsible for crimes : Detectives are investigating the death of a baby boy. | Detective Inspector John Hartwell
a plain-clothes police officer wears ordinary clothes instead of a uniform : Two plain-clothes police officers, acting as hotel security men, kept watch on him.
a British police officer of the lowest rank : a police constable | Constable Robin Cameron
a senior police officer who is in charge of the police in a particular area in Britain : the chief constable of North Yorkshire police
a police officer : You’d better call the cops.
a US police officer in a state police force : a New Jersey state trooper