what happens if two different sperms fertilize the same egg?

Only one sperm can fertilize an egg, even if two different men ejaculate into a woman. As soon as one sperm cell penetrates the cell wall, the egg immediately undergoes a chemical reaction that prevents other sperm cells from penetrating.

Why would the timing of ejaculation have anything to do with it? Fertilization might not happen for days after sex.

On the rare occasions that one egg is fertilized by two sperm, do you know what you get? A large tumor, not a human baby with three parents. This is called a partial molar pregnancy. There are pictures online, if you have a strong stomach.

Think about the logistics of getting two penises into one vagina at the same time. And no that doesn't happen

Two sperm CAN'T fertilize one egg! Take a biology course for Pete's sake!

HOWEVER....if there are two men who have sex with the same woman - even within a few days - if the woman "drops" two eggs, there IS a possibility of twins...and both men could be the father of one of the babies.

Exactly how does a woman have vaginal sex with two men simultaneously?

It has happened that a woman has had twins from two eggs that were fertilised by different men that she had sex with within a short period of time (not simultaneously). If two sperm fertilised one egg the embryo would have triloidy - three sets of chromosomes instead of two. The embryo would not develop beyond an ealy stage and would be miscarried.

It wouldn't fertilize the same egg but two different eggs. She would have twins but they would have different dads. The reason this is even possible is because some women have two uterus.

Nope .. that's not going to happen. Once an egg is penetrated by ONE sperm, no other sperm can (shall we say) 'get into' it. That's nature's way of preventing the very thing you are talking about.
In the 1 in 2000 chance that something like that DID happen, the embryo would have 3 different sets of chromosones and would be grossly abnormal. The woman's body would just absorb the abormal embryo very shortly after fertilization, or it would be miscarried a month or so later.

Read
http://www.bbc.com/news/magazine-28986843
... having three biological parents IS possible - but not in a normal way.

You'll notice the DATE on that article; the treatment has existed for several years.

Except that lazy journalists seem to have missed that news;
http://www.forbes.com/sites/emilywillingham/2016/09/28/controversy-erupts-around-baby-with-three-biological-parents/#47c4a17577a1
according to this Forbes article it's a new procedure. Silly Forbes ...