Do you mean a double radiator with one valve at each end? If so then look for a bleed valve on the cold one. They do have one each. If bleeding it doesn't work then the thing is blocked and the water is taking the easier route through the other one. I can't think of any reliable cure other than taking the thing off and washing it out. If you've never done this before be prepared for an outpouring of black sludge that stains everything it touches.
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