The veteran midfielder stepped off the bench in the 69th minute with his side trailing the leaders 1-0, but he held his nerve from the spot twice in seven minutes to turn the game on its head.
Posh, who had three other penalty claims waved away by referee Eddie Ilderton, held off a late barrage of pressure to claim a vital three points which moved them out of the relegation zone.
"I wouldn't want anyone else standing over a penalty," Ferguson said. "Grant has a terrific record and put them both away superbly. We could probably have had six penalties in all.
"We have beaten a team that is bound for the Premier League twice - and we have done it on merit on both occasions.
"Hopefully it turns out to be a big win for us. I'm not fussed about our league position because it is points that matter to me.
"March has always been a good month for us and that has certainly been the case again."