What are the issues with pulling power off a single battery while it is wired in series with a second battery?
Yes, you are better off getting a dc to dc converter to convert the 36V to 18V to run the 18V motors.

This is because you will cause an imbalance between the two batteries by using only one to run the 2 18V motors.

You could, possibly, consider running 1 18V motor on each battery but that may mean control and wiring issues and the possibility someone makes a wrong connection... Lots of sparks or a fire then.