Impossible? have you seen the current rankings? Rafa is only 900 points ahead of Federer. With so many points to defend over next 4-5 months, Rafa will have to work VERY hard to keep his #2 ranking. It is very likely that Roger will grab #2 ranking from Rafa, at least for a short while.
it's not that likely : after Wimbledon there's a correct chance, but for the seedings in Roland-Garros and Wimbledon it's quite unlikely considering what Nadal has constantly done on clay since 2005 and that Fed will probably not play Monte-Carlo and Barcelona.
It's enough for Nadal grabbing 900 points from Fed before Roland-Garros, and then for Wimbledon seedings, he will get 2070 points more than Fed thanks to the specific rules to Wimbledon, then it's also unlikely that Fed gets number 2 seed in Wimbledon.
But later for the Olympics and the US Open the possibility is higher of course, esp. if Fed does well in Wimbledon, and later in the Olympics and the US Open.