I have to write a note to my course teachers and both of them are women. So, how should I address them? Dear Ma'ams or Dear Madames or anything else?
If possible, you should use their names. It is generally rather impolite to use "Dear Madam" when writing to someone whose name you know, or whose name you should know. If you don't know their names, make an effort to find them out. For a formal letter you should use last names with an appropriate title (Ms or Dr or Prof. are common titles for women teachers)
Dear Ms Smith and Dr Jones,
"Ms" is correct and neutral. "Mrs" is correct if you normally refer to someone as "Mrs". "Miss" is usually avoided in a formal business letter, but can be used if you have been asked to by the woman to use this title.
Structures like "Dear Sir or Madam" are used in cover letters. These are letter that are sent to an unknown person who you do not know and are not expected to know, for example when applying for a job you don't know the person who will be hiring you. As you don't know the person, you don't know if they are a man or woman. It is very unlikely that you will know someone's sex but are not to know that person's name.
For a casual note, use whatever you would normally refer to them by.
Dear Katy and Nancy,
In the unlikely situation that you know the sex and number, but should not know the name of your teachers, I would use "Dear teachers," or if not teachers then "Dear Mesdames". To be honest this is very very unlikely to ever actually occur.