Most commonly a secondary radio will be provided to the crane operators, so they can inform when they need to work within the other cranes radius to the other crane.
There are crane anti collision devices such as the T-3000 from e-build, this device warns when working with the other cranes working radius. I have never personally had any experience with these devices but my understanding is that they are secondary safety devices and the crane operator still must be alert to the risk of collision
"TAC-3000 is not a fail safe safety system as the tower crane operation is still in the operator's control."
Of course other cranes are not the only risk of collision on site, other moving machinery such as concrete boom pumps and excavators are machines that work within the crane operating radius that the crane has the potential of colliding with.