Make two joints in an IK chain rotate the same

[littonce]

So I have an IK setup like this:

Double-jointed elbows and knees. Is there a way to make the two joints in the middle rotate the same?

[mcw0]

By "the same", I'm assuming you mean the same rotation angle?  You can setup a normal 2 joint ik chain to drive your 3 joint chain.  To achieve the same rotation angle, just adjust the length of the second joint in the 2 joint chain.  Simple equation for a triangle should solve this for you.

cos(C) = a^2 + b^2 - c^2 / 2 * a * b

C is the angle opposite side c of your triangle.