I think Carrie is headed in the right direction...Make sure that you are showing your cockatiel that he is the Alpha bird, or top bird...He should be first for everything. He should be getting the most attention, but don't over do it...try to spend the same amount of quality time with him as you did before the quaker came.
When you play with the quaker I would recommend that at least for now you take him out of the cockatiels sight while doing so, so it doesn't promote more jealousy.
It could very well be that the body language the quaker puts off could be intimidating to a cockatiel and that has him worried. So you might also try putting the quaker in an area where the cockatiel can't see him as well.
It does sound like your cockatiel is mad at you because he's turning his back on you...its showing that he doesn't respect you and his feelings are hurt. Give him time...and show him the top bird respect that he should have since he was first in coming to your home.
Eventually, things should even out and he'll come back around to being your buddy, hopefully.
Stop and think about the situation and put it into human terms...you have a two year old child and all at once you bring a baby home with out preparing the older one for it...you fawn over the baby and don't give your undivided attention to the 2 year old any more...What happens?? The two year old gets jealous...pouts for a while and then will probably act out to get attention because even negative attention is better than no attention in their eyes but eventually things even out and all is well with the world. The birds will probably not be buddies but will be able to tolerate each other in supervised conditions.
There's more things you can try but the ones previously mentioned should help.
Let the cockatiel out of his cage first and longest...put the quaker back before you put the cockatiel away when they have out of cage time... Even if the cockatiel has been out of his cage for longer time, if you put him away first that's showing that he's lower rank (IMO) than the quaker. You want him to feel that you still regard him as your Top Bird, your main squeeze

Good luck...