I have user-a and user-b on my box.
How do I do a no-password
su - user-b as user-a?
su is not meant to do that --
/etc/sudoers.d/custom and write the following:
user-a ALL=(user-b:user-b) NOPASSWD:ALL
Which means: whenever user-a executes
sudo -u user-b (or any other variant), let him go without asking for password.