we have ds age 14 plus 2 younger siblings.
there is a PS3 that they jointly own.
ds now has an x -box which is his.
They are plugged in to the same TV in our dining room.
I have no screens, TVs DVD players or computers in bedrooms. This is actually the advice from any organisation to do with internet safety etc. We have heard it from multiple sources.
Phones allowed in rooms, but not overnight.
We are lucky that we have a dining room we can use, but I also use it for tutoring, so no x-box/PS3 until I have finished.
I think you need to allow your eldest to have his own x-box without making him share. If he does, great, if he doesn't then that is his choice.
At the same time, you should find/make space for your younger one to have some kind of console. (or, decide he can't have one until same age that ds1 had one, and stick to that) If, like at our house, they have to share a TV screen, then they have to work out either joint games, or some kind of rota. I let mine work out the rota between them, and refuse to get involved. It causes long discussions, but in the end it is written down and stuck to the wall, and they work it out. They know that if they don't stick to it, they all loose out.
I have had to change some of my rules to make this work, eg youngest gets in earlier than older two, so she is allowed on then, so that they can go on later. It also means none of them can be on for too long at a time, as they have to take turns.
You can also have one device and 2 rules, eg ds 1 is allowed x-box live, and ds2 isn't.