Sometimes grammar references told us that you cannot put a comma between the two verbs of a double predicate sentence, but I often found that usage in articles in famous websites or newspapers. Could you please tell me whether the following sentences are right or not and why. And another question is whether the two verbs in a double predicate sentence can show two difference tenses. Thanks for your help. 1) I was passing by, and notice your posting. 2) The police have installed 2.75 million surveillance cameras since 2003 and are expanding the system into the countryside 3) we understood to mean that she had reached an impasse, and needed to be lowered.