Can you be a little more specific?
You haven't given a lot of information, but it sounds like you are senior man to these two. You have every right to approach them and discuss these issues. Do it as an individual. Since you are not, at the request of your senior staff, making these inquiries, let them know that. If the answers are not satisfactory, inform them that you are going to take it up with the department's leadership. But make sure that they know that. That way, it doesn't split the department and there is no perception that it was done behind someone's back. If it is common knowledge that these two are going to the town council and complaining, then why hasn't the chief intervened?
If they have violated your department's policies and procedures and even your chain of command, why hasn't something been done?
I believe that I have asked more questions than I have answered, but you didn't give me alot.
In any event, derisive behavior needs to be addressed or it will hurt the department.