First set up IK for one arm, and then when you create IK for the second arm you choose the same IK target.
Alternatively you can create IK for each arm and then use a transform constraint to constrain one IK target to the other. Please note that if you do this you will have to change the order of which the constraints are calculated. In this case you want the transform constraint to be calculated BEFORE the second IK constraint. Do do this, drag the transform constraint up above the second IK constraint under the constraints node.
Loading Image
I hope it helps.