AFAIK, if iframes come from different domains, this is not doable in a “common” way (unless you control both iframes), browser security measures prevent it, otherwise you could wrap a banking website and steal passwords, for example, when you log in.
Some info on workarounds: http://softwareas.com/cross-domain-communication-with-iframes
If you control both iframes:
Drag & Drop between iframes by sample: http://www.dhtmlx.com/docs/products/dhtmlxTree/samples/05_drag_n_drop/06_pro_drag_frame.html
Cross Browser HTML5 Drag and Drop: http://www.useragentman.com/blog/2010/01/10/cross-browser-html5-drag-and-drop/
Another sample: http://jqfaq.com/how-to-implement-drag-and-drop-between-iframes/