no way. I'm sorry but that's completely unreliable. Every marth I've played will more often than not short hop double fair, which is mostly safe on block when spaced and incredibly hard to spotdodge.
Spotdodge?? what?
Let me run through what would happen if you try to spotdodge marth's fair other than for the rare surprise maneuver.
1) spotdodge the first one. Get hit by the second one in the sequence as you come out of the spotdodge. No?
2) 1st fair misses your shield, he goes in for the second one. you can spotdodge and you might be able to d tilt, or more feasibly roll away to reset the situation.
3)you shield the first hit, spotdodge the second. in which case you probably don't even have the frame advantage to do anything other than buffer a shield roll or a jab. And the jab probably won't have the reach to hit him anyway, that is if he hasn't started throwing out D tilts or something as you were coming out of the spotdodge lag.
Whatever you do, don't let the first fair hit you, I'm pretty sure double fair is a true combo on Ganondorf unless he techs, because I've been juggled out of short hop double fairs across the stage many, many times.
What you want to do is shield both hits, roll away. He can chase this with more double fairs though - so don't be predictable.
Now here comes the part where the player Makes up for Ganon's shortcomings. Double short hop fair is **** easy to punish if you
know he's gonna do it before-hand.
Let both fairs hit your shield and see what he does. Does he D tilt? next time jump out after both fairs hit and thunderstorm him while he's trying to poke your legs.
Better yet, predict the double fair entirely and wiz kick him in the face after he leaves the ground- it goes through/between the fairs almost all the time.
But, this being brawl, you're going to have to go the defensive route on this one. Shield that *****, seriously, and then try to predict it next time.