does that to me also
i don't think there's a way unless you have another computer or use 2 different browsers

There isnt a way too do that , there linked , Nothing can be done.

I had just run a search on this issue , And found Plenty of people with the same issue.

Not sure , Only thing i can really say is "noone and nothing is perfect" , Its weird but , Theres a full forum page of people asking how too unlink them

its probably because it replaces the cookies each time you access another account since youtube and gmail use the same cookie (i think) :/
as i said, use firefox for one and IE or another browser for the other