You have to look at it as an individual setup for each motor. Each motor has it's own independent ESC. So if the current draw of one motor is 18 amps, then that motors' ESC needs to be able to handle the 18 amps independently. Each side is it's own, completely seperate setup.

Now, if it is a combined draw of 18 amps, so 9 amps per each motor individually, then yes, you could get a 10 amp ESC for each side. It is always better to have some room rather than just barely squeaking by. The main advantage is less stress on the system by going over on the ESC amp rating. It stays cool and will last a long time.