Hi Fred, welcome to permies.
Plant autosterility is related to genetics. When a seed is formed, the "egg" is in the female flower and if it only allows an unrelated apple tree to pollinate it, then the pollen ("sperm" equivalent) must have come from another tree. All the seeds in a single apple will have the same "mother", but there's no guarantee that they will all have received pollen from the same "father", but since apples tend to be bee pollinated, it's likely. That said, there's a reason my spouse refers to it as a "gene splicing experiment". There are two sides (they teach about the "double helix" in biology classes and that's where it gets involved) and different seeds can get different sides when the division is happening. So considering how long it takes an apple to grow, I would be inclined to grow more than just two seeds for two reasons: one to be *sure* that you weren't unlucky
enough to get exactly "side 1 + side 1" in both seeds, and two because although permies *really* encourage people to raise plants directly from seeds, apples are known for doing what is called "not breeding true". That means that you will get an interaction of characteristics from both parent trees and the results may be an apple that tastes wonderful or not, be good for some purposes or not, be softer or harder, ripen earlier or later or any other characteristics that humans look for in domesticated plants. Having more than two trees to choose the best ones to keep with less successful ones being used for building soil,
firewood, feeding animals etc, may be the best strategy if you can manage it.
The short form is - statistically, the two offspring should be capable of pollinating each other, but if you're only planting two trees from a single fruit, there is no guarantee.