Tyrion reflected that his father should have been able to defeat Robb in the west before Stannis could have taken Storm's End. Wasn't this move a great risk though, since Stannis could have abandoned Storm's End at any time, in order to strike against King's Landing and the Lannister claimants to the throne while Tywin was occupied in the west?

Storm's End is a hugely formidable castle, and should have been able to hold out much longer, as it did during Robert's Rebellion when Stannis was inside rather than outside. And both Tyrion and Tywin knew that Stannis was a methodical commander rather than a daring one, and therefore would be unlikely to leave an enemy stronghold untaken in his rear. There was also the psychological aspect, as Stannis himself explains to Davos; he could not risk being seen as having suffered a "defeat," however minor.

Was Lord Tywin marching west a huge risk? Of course it was. That was why he sat at Harrenhal for so long, hoping to lure Robb into attacking him... or Stannis into committing against King's Landing. Neither of his foes would play into his hands, however. At which point he made a calculated gamble.

In a three-sided struggle (four sided when Renly was still in the game), any decisive move is a risk... as I learned in high school playing... yes... RISK! But the only way to win is to take some of this risks.