I think if they tried enough times they probably could. With two weeks of training? I doubt that would be enough. I really hope they play together some time just for fun, winning anything would be a bonus, it would be great publicity apart from anything else
Pretty sure it WILL happen one day. Nadal has asked Federer twice already, and while Fed declined because of scheduling reasons and the like, he did comment that he found it a very nice idea. Should the time arrive that the two of them will find themselves sliding on the rankings for a bit (hopefully, not the coming couple of years
), Fedal taking part in a doubles tournament wouldn't surprise me at all, if only as a 'gimmick'.