Uke is holding your one wrist with two hands; hit him!

Of course that is what we should think, but we atemi instead. So, with your free hand (neglecting footwork for this discussion) atemi to his face. If he is a goof, he will hang on to your wrist and take the hit to the face. Why do we practice like this? It makes no sense. Sensei says that if the atemi hits Uke in the face, he will let go anyway. So, Uke should release one hand to protect his face and leave the other one attached.

If he lets go of his front hand, it becomes kosadori and a kotegaeshi or even a jujinage is tempting to do. If he lets go of his back hand (normal!) then you can work the technique just like a one-handed grab.

If Uke is stiff and refuses to let go, after the atemi you can slide your tegatana down his arm with moderate pressure. The idea is to scrape his radial nerve with your teganata and put constant pressure on it to weaken him on the way to capturing his hand. This is Aikijutsu, though.