2 devices on 1 live gold membership

These two options are very good contenders. If the person you're trying to share Xbox Live Gold with, as well as any other digital content like games is a member of your family... using the family features would be a good move.

If they're not related to you or are old enough to not need to be integrated into your account's family setting, then the 'Home Xbox' sharing would be good a move instead.

Here are the details on both.

Family Settings: https://www.xbox.com/en-US/community/for-everyone/responsible-gaming

Home Xbox: https://beta.support.xbox.com/help/hardware-network/console/my-home-xbox

Make a family account in your xbox settings.

Yes you will have to set the 2nd xbox as you home xbox on the main account with live active

What will happen to my 1 year live gold membership? I bought 1 year live gold membership.

Dragon Age 2 was free with xbox live gold membership from Dec 1 -Dec 15. The xbox game pass is a different membership that gives you access to a list of games and is not related to xbox gold.