Hello and welcome
to the forums!!
Your birds are stunning and I'd love to see more of them
To attempt to answer your question, Fallow (as you probably know) is a recessive gene, so to get any fallows at all, both parents must be fallow. If only one of the parents is Fallow (assuming the other is not split Fallow) then all the babies will be split Fallow and therefore not show the gene. Because Crested is a highly variable gene, I would breed two visual Fallow parents (as opposed to one visual and one split, or two split) because if all the chicks are fallow, that would give you a better chance of getting a fallow and crested budgie.
Crested budgies apparently are in possession of a very strange gene, which, although autosomal dominant, only acts dominant in certain cases. If you have a DF crested, it will be crested, but if you have a SF crested, although it still has the dominant gene, it only will show the gene 15-20% of the time. So you could have a bird that visually looks normal but actually is SF crested.
Apparently, the best way to breed for a crested with the best possibility is to take a known DF crested (either from two visually crested parents, or with very noticeable cresting) and breed it with a SF crested, or a normal, both of which should be the result of crested parents and both which are indistinguishable from one another as the gene appears to be lacking entirely if the SF crested does not express the visual trait, much like the normal offspring of a SF and SF crested, which could be a normal.
So for a breeding pair, I would select two fallows, one which is DF crested (ensure this by looking at lineage, if both parents are crested there's a good chance he's DF) and one which is SF crested, but visually normal. Basically any chick from a crested pair who doesn't show a visual crest. They could have one gene, making them SF, or zero, making them genetically as well as visually normal, but unless you knew the exact genetics of the parents it would be almost impossible to tell.
Although you could breed two crested budgies together, by breeding it with a visual normal it cleans up the genetics a bit so it's not as cluttered, similar to the reason you don't breed two inos together.
However, both parings will yield crested budgies, DF x DF and DF x SF. With both parents being fallow you are almost guaranteed at least one fallow crested, if I'm not mistaken.
So I hope that helps, that's what I could find on crested budgies. This sounds very confusing when I read over it
Hope to see more of you around here, it's great to have you with us!