How to sync two ipods to one mac? so, my sister and I both just bought the ipod touch 4G. She installed and downloaded her apps from the Itunes. I've purchased some items in my ipod in the store where I bought my Itouch. Then, the problem is that when I try to transfer purchases of mine to the Macbook (that my sister use), 

it kept give a warning "Some of the purchased items on the Ipod "my name", including "name of certain apps", could not be transfered to your Itunes library because you are not authorized for them on this computer"

Then, I try to choose "Store > Authorize this computer" with my Apple ID. the next warning pop out "This computer is already authorised"

So, how to sync my apps to this Itunes?
Are you using the same login (system user account on the mac) as your sister?  The easiest thing would be for you to create a 2nd Mac (user) login, and then associate the iTunes account under that login with your Apple ID (not your sisters).