Not to get all Occam's Razor here, but the only life form capable of creating eggs was on the Sulaco.
There were two times where she was off camera: before she emerged from the landing gear and when she was searching for Newt.
If Batman can continually "disappear" when he's off camera, I can buy the Queen Alien sticking an egg somewhere while no one's looking.
And since this was a second or third generation egg through Earth organism impregnation, I can buy the idea that the xeno has adapted to allow two embryos in an impregnation. Human and canine (or ox, depending on the version) would not be very different from each other, from the perspective of the Xeno. All three are from the same mammalian evolutionary branch. Hell, I can even buy the idea that multiple impregnations have always been possible.
Anything else is just over thinking the issue, in my opinion...but it is part of being a fan.
