The Two On One, often referred to as The Russian, is a very under utilized tie-up in the younger ages of our sport. It is more often seen in college than anywhere else. As always, if we can get our young ones to start doing this, they'll be years ahead of their peers.
As you can probably get through the name, we're controlling our opponent's one arm with two hands. Both palm up, one is one the wrist while the other is holding the shoulder. This gives us a good angle to do a wide variety of shots.
It can be part of Hand-Fighting and is not meant to be held onto for very long. With tie-ups, we feel you should use them as soon as possible, or move to the next one. This is often based on how our opponent reacts to whatever tie-up we currently have.
The easiest way to teach this is to have your partner post on your left shoulder with his right hand. You can then grab his wrist with your right hand palm down, and rotate it down so that your right hand is palm up. Your left hand will then fish hook the underside of his right arm pit. I like to try to get as high as I can in the arm pit to give me as much control as possible over my opponent's upper body. So you have two hands controlling one arm (two on one), both should be palm up, and you'll be out to the left side a little bit.
Usually when you get the Two-On-One, or any other tie-up for that matter, your opponent will want to defend. The smart will find a quick shot, using offense as his defense. The less informed will try to get free and escape the tie-up first before doing anything else. For whatever your opponent might want to do when trying to escape the Two-On-One, we have an answer.