wouldn't polyembryonic mean that it has two or more embryos? That would mean that it would put up two or more sprouts, right?

Most seeds seem to have two sides that split, don't think that makes them polyembryonic.

yeah, this does not look polyembryonic then.

There's probably a great chance these mangos will taste like crap then, if they survive.

Not necessarily, it's more like the roll of a dice ...... you don't know what you're going to get. It's not considered worth it because it takes many years to find out whether the dice fell the right, or the wrong way.

To give you an idea this is what polyembryonic look like. I just found out it was so yesterday when it sprouted.

Do you guys know how will this seedling grow with two or three stems? How to separate later on?

I don't think you would separate them; pick the strongest and pinch off the rest.