My FD seeds seem to be producing a lot of (dual offspring) per seed. One is always much larger than the other. I believe one is the zygote right and that is the hyrbrid seedling? Right now I'm keeping both, but beyond growing one healthy tree for future fruit, the rest of these seedlings are intended for me to experiment with grafting. What is the best practice to follow with these?

I find that when I prick them out of the germinating tray into their own pot, they catch up quickly with their "twin". I still use them. I find that just having them for a rootstock that is cold hardy and good for my sandy soil makes them acceptable for what I need them for. The little I may lose on the height makes no difference to me.

So the twin, is going to be the same as the sister seedling? I don't get citrus reproduction yet.

Reverse, You can't assume that the larger or the smaller one is the zygotic seedling. In most cases, the zygote died long ago, and you're looking at two nucellar seedlings. Seedling size, at first, is based on how much of the seed that embryo (with its bit of food-holding cotyledon) occupied. Once they have grown for a while, on their own photosynthetic power, then you can rogue them more successfully, if you choose to. And in that case, the true FD seedlings will tend to be shorter, with much more curled thorns. Also, specifically having just 2 seedlings is not a constant or necessarily typical thing -- you can get from 1 to 7 (maybe more??) from a seed. Each variety will have a characteristic average, but with much variation from seed to seed. But I'd think the production of precisely one nucellar and one zygotic seedling would be rare.

I didn't mean to infer that the second embryo was any particular type, that is why I enclosed it in quotation marks. Sorry reversethong....

In the 100 or so seedlings I have germinated, not a single one has been the true FD. I understand the true zygote seedling is a rare one....few and far between, so to speak.

Out of the last two batches of 50 seeds set, only two seeds from each set produced two embryos each. The rest were single seedlings. Now explain that! Exactly 39 germinated from each set of 50. I found it odd that the numbers were the same. Is this common?

The latest batch I'm germinating has produced 7 seedlings out of 4 germinated seeds.