I'm using Google Chrome on a Chromebook, and I started having this problem about a day ago. If I have two (or more) Youtube videos up in different tabs, I cannot play both at one time. Whenever I click "play" on one video, it'll start playing like normal, but any videos I have playing in other tabs will automatically pause. NOT buffer or freeze - actually pause, and I'll have to go over to the tab and manually unpause it (which will pause the other video). It's really annoying, since I like to listen to music while watching art or video game videos Any idea why this could be happening?
I use this extension to make the video pause if I move to another tab (explained below)... it may help you unlock the Youtube setting. you may have to enable the setting in the extension and later disable it*.
These are the settings I use in the "YouTube™ No Buffer (Stop Auto-playing)" extension:
(✓) Stop auto-buffering of embedded YouTube players
( ) Stop auto-buffering the player on play-list mode
(✓) Release no-buffer (start playing) when tab gets focus1
(✓) Pause video when tab is hidden2 *(Enable this setting, restart the browser and disable it)
1: If this option is checked and  is unchecked, YouTube player is released from no-buffer mode once.
2: If this option is checked and  is checked too, YouTube player gets paused when tab is hidden and it gets resumed when tab is active again.