It sounds as though both account holders are sharing the same PC/laptop etc? If so then this can happen when one fails to clear the cache at the end of a browsing session and the other account holder doesn't notice.. A similar thing will happen when two sellers share a machine: listings can end up on the wrong selling accounts. Ideally each user should use a different browser.
Exit your browsing session, clear cache & cookies and then log directly into your Paypal account and check the account funding settings; reset as necessary. You won't be able to do anything about the payments already made but you should be able to stop the problem recurring.
If you're still having problems, come back and another responder may be able to provide more help.
