This is usually a leverage problem. You can't get sufficient arm strength on the two wrenches simultaneously.

The top nut must turn counter-clockwise and the bottom nut must turn clockwise. See if you can get the wrenches so that the top handle is slightly clockwise of the bottom handle. Then, you can squeeze the two handles together with one hand. This will immediately break them loose as all the force is directed at the nuts and you've got perfect leverage.