You can transfer accounts from one email address to another by simply changing the email address on your account! You can reference the article under 'See also' for instructions.
However, combining two accounts or separating one account into two is not possible. If you would like to change your email address to an address associated with a different Khan Academy account, you will first need to log into that account and delete it or change its email address.
Can I change the email address I use to log into my account?