If you bred the satin buck to the longhaired doe all the babies would be satin carriers, so I would pick a male and female out of the litter with the longest coats and breed them together. You would get both satin and standard in longcoats.
Satin and longhair are both recessive, which means they have two copies of the gene for it to show (one from each parent). If you breed a satin to a longhair, assuming they are homozygous, the litter will all be normal coated mice but carry satin and longhair. If you then breed babies together they have a one in sixteen chance of producing a baby that is both satin and longhair.
I'm playing devil's advocate here, but I have to ask--why do you want satin longhair mice anyway?
Satin results in partially hollow hairs (which is why the light can shine through them and make them shiny), and angora/longhair elongates the hairs which means they break just a little bit easier, especially if they're satin and hollow. What this amounts to is that angora satins (especially females) are often indistinguishable from plain satins. So if you had a bunch of angora satins, you might not necessarily even know it.
Here's a pet-type mouse of mine that I bred a few years ago who was angora satin:
You wouldn't be able to tell she was angora by looking at her but both her parents were and all her siblings were.
hmm maybe it just depends on the genes? because ive seen some really nice looking long haired satins before. maybe they are just difficult to get right? i was just going to breed long haired mice but i really like how satin look on them so i thought i would try it out. maybe they will end up looking like normal satin. and if they do then i'll just go for regular satin and regular long hairs