OK So this is a really old question. Thought I would respond since someone else might find this question via google as I did.
1. Constain Angle 1. Leave the dimension at whatever dimensionyou happened to draw it at.
2. Make the dimesion driven. (To do this click on the dimension, click on the driven button [button next to the button that makes a line a construction line])
Notice you can now just manually change the angle. and the dimesnsion on shows what it currently is at.
3. constain the second angle. At the input stage, select the first angle.
I wanted to do this with 2 requirements:
1. No additional dimension.
2.Fully constrained sketch.
My solution is to make identical isosceles triangles with construction lines, one for each angle. One of the 2 equal sides is constrained to be equal to some random, relatively short line; maybe one of the legs of one of the angles. Then for each angle, I make the equal sides of a triangle collinear with each leg of the angle. It can be a bit tricky not making everything deform while you're adding constraint, so adding a few dimensions and changing them to bring things closer to collinear can help.
I actually played around with this a bit and I think I like the appearance better if I put the construction triangles on the "outside" of the angle, and make the sides perpendicular to the angle legs, rather than collinear. It also seems to make it easier to get the constraints on without contorting the figure into a twisted mess. Or maybe I'm just getting better at that.