If you sign out of your Gmail on any device, it will sign you out on all devices! Simple!
But you still have to sign in on multiple devices for security reasons!
The thing is, I wasn't signing out. It was just signing out on it's own, while I was in the middle of doing something. ugh!
(i think I somehow found a workaround, but yeah it was weird)
Use multiple web browsers; chrome, firefox, torch... also in each of these browser you can open a new incognito or private browser window that will not save web data. So by example, with those three browsers, you should be able to log in to 6 different accounts at once. 3 accounts that could be saved logins and 3 private sessions that won't be saved at all. Also you can open multiple incognito windows so it might even be unlimited private sessions, never tried it before.
i'll have to read this again when I'm awake. haha! but this might be good for if this happens again. (i think? going to bed now. :P...also, I like your avatar/profile pic! :P )