Let's take for example an Ogre in a 10' wide corridor facing two fighters side by side in front of him.
The ogre wants to perform an overrun attempt to pass over the two fighters and get to the wizard behind them, both fighters block him, thus resulting in a opposed overrun check.
How do you resolve that overrun attempt? From what I understand there seems to be two options.
Since an overrun attempt is a Standard action made as part of a move, you cannot make two of them. So the ogre picks one of the two fighters, and if the ogre wins, the ogre must squeeze past the second fighter that is still standing to get to the wizard (thus resulting in all squeezing penalties on movement, AC and to hit rolls).
Since an overrun attempt is basically the creature moving its space into the opponent's space, in that case the ogre occupies both fighters' spaces and thus must roll and beat both fighters' opposed checks in order to pass them and get to the wizard.
To make the overrun attempt according to the rules, the ogre squeezes to enter one Medium fighter's space—provoking an attack of opportunity from the fighter—then follows steps 2 through 4 of making the overrun attempt conventionally, all the while still suffering the penalties for squeezing until the ogre exits the fighter's space. (The other fighter may have the chance to make some attacks of opportunity in there, too!) The typical ogre can't make a second overrun attempt or make two overrun attempts simultaneously. (The previous scenario assumes the DM even allows voluntary squeezing when actual obstacles aren't about, an issue addressed in this question.)
This conundrum doesn't really have anything to do with the attacker not having enough standard actions, though, because a defender that during step 2 that allows an attacker to pass through doesn't cost the attacker an action anyway. Instead, it stems from the troubling line at the beginning of the description of the special attack overrun: "You can make only one overrun attempt per round" (Player's Handbook 157). Done. Full stop. And, to be clear, an attacker has still made an overrun attempt even if the attacker encounters a defender that proudly lets the attacker pass through!
Whether this strikes a particular DM as a problem isn't for me to say. This DM has considered making a house rule so that, for example, an elephant can overrun a squad of Medium dwarves or whatever, but the the enthusiasm necessary both to concoct such a house rule and to playtest the rule to ensure its fairness has never materialized. The special attack overrun just gets filed away as (ahem) largely useless therefore rarely used.
Note that prior to the 3.5 revision, overrun was something that happened exclusively during a charge (see here on Combat on Combat Actions). An overrun was just a means to get past one defender that was in the path of a charging attacker's preferred target… and it was still only usable once per round (so even prior to the 3.5 revion, the ogre'd still be out of luck). It was the 3.5 revision that made overrun a distinct special attack, removed overrun as an option during a charge with errata (errata that remains unadopted by Pathfinder, by the way), and even added an Improved Overrun feat to the core rules, yet, all along, seemed to forget that creatures bigger than Medium exist.
It's option 2, it's basically the same concept for flanking a character whose size is greater than yours (more character can flank him).
Here it's the same, because the ogre is taller than the two fighters, when he tries to overrun them, he will have to beat the combined score of the two fighters.
It's to actually encourage teamwork for taking on large or monstrous size creatures.
Opponent Blocks? If your opponent blocks you, make a Strength check opposed by the defender’s Dexterity or Strength check (whichever ability score has the higher modifier). A combatant gets a +4 bonus on the check for every size category he is larger than Medium or a –4 penalty for every size category he is smaller than Medium. The defender gets a +4 bonus on his check if he has more than two legs or is otherwise more stable than a normal humanoid. If you win, you knock the defender prone. If you lose, the defender may immediately react and make a Strength check opposed by your Dexterity or Strength check (including the size modifiers noted above, but no other modifiers) to try to knock you prone.
I checked if something was actully specified for a multi-block, but there is none, it's up to the GM, so you have to choose if yout want to combine the dice score of the two fighters or if you want to make them roll in order.
I'm actually more prone to make a combined throw for the two fighters if the total space they have is less than the ogre: for exemple, your fighters are each 2feet large (it's an exemple) and your ogre 5feet, then you combine the dice score. But if they are larger than the ogre (like 2.8' each) then you make the ogre choose which character to overrun.
you use the same method as the flanking rule to decide:
When in doubt about whether two friendly characters flank an opponent in the middle, trace an imaginary line between the two friendly characters’ centers. If the line passes through opposite borders of the opponent’s space (including corners of those borders), then the opponent is flanked.
But here you actually check if the fighters both fit in front of the ogre. If not you choose.