You need to transder any accouts you are sitting before yours can be sat. I dont know what happens to an account you are sitting if you don't pass it on first, whether it is simply inaccessible until your accout is no longer being sat, or if you lose the ability to access the account altogether even after you return to your own.
Not for me, everytime I've been sitting a few peoples account and needed sitting myself, once I come back and end the sitting mode, the original accounts I am sitting are still being sat by me and I can still access them.
"You are right now sitting one or more accounts. Your account sitter will not be able to access the accounts you are sitting. Are you sure you want to send out the account sitting request?"
That is what comes up and you have the option to click 'OK' or 'Cancel'.