It seems to depend what order you do things in.
If you are signed into YouTube with account A, then switch to account B in GMail, the next reload of YouTube will assume you want to sign into it with account B also.
What you have to do it either stay in 1 email account whilst using YouTube (as long as you don't switch you can use two accounts at once... if you sign into B for GMail, then go to YouTube and switch to account A you can use both simultaneously... also you should be able to switch to A in GMail again without any YouTube problems, but if you then switch to GMail B again it will try to switch you in YouTube too (obviously you don't have to, and you can use the YouTube switch account feature to get back to A, but if you need to do that a lot it will get annoying) Or the other option would be to use separate browsers (maybe use 1 browser for the thing you are likely to need a lot of account switching in, probably email, then just use you main account on your main browser)
I didn't realise there were still problems like that happening with this multi account shit, I lost access to my original YouTube account when they first joined with the Google account, I couldn't remember my password and it wouldn't accept my email address to let me change the password... I think that may have been because I also had a Google account with the same email address. Anyhow, I managed to fix that problem last year
I actually have 4 Google accounts now... my original one I was forced to set up to be able to use blogger, I'm not entirely sure if I even have GMail in that Google account because it was set up with my Hotmail address (that is also the account now tied to my original YouTube account). My second Google account is for my old work email, Which prompted me to start using Gmail over Hotmail as my personal email, so my third Google account (which is my main one) was created shortly after that. And I also have another GMail account I set up mainly to get around some of the new security rules Facebook and Spotify use to stop people having multiple accounts.
