Difficult, since a hole that a pig can fit through will probably easily admit a sheep. I'd say your best bet, apart from raising them separately, is to fit each sheep with a poke. Styles vary, but basically it's a neck yoke with some kind of wooden protuberance to catch on a
fence to keep the animal from getting out. (Drew Langsner's book "Country Woodcraft" has a chapter on pokes.).
I'd think a creep with a vertical opening, just wide
enough to admit the pigs at their fattest, accompanied by sheep with pokes featuring horizontal shafts (sort of like a set of prosthetic horns), would do the trick. But seems it'd be easier to just raise them separately.