We have bred wild-caught hybrids between blood pythons and Sumatran short-tailed pythons (verified by a DNA examination) and had very low fertility.
We've also bred spotted pythons to Childrens pythons, for 100% fertility, but bred the hybrid babies and had close to 0% fertility.
Personally, I'd guess that a cross between a Python curtus and a Python breitenstein would result in a good clutch of hybrid babies that themselves are infertile or have very low fertility.
People, these pythons are now recognized as different species, and I believe rightly so. They aren't minor geographic variations on a theme. They are different species.
Crossing a Sumatran short-tailed python and a Borneo short-tailed python is pretty much the same as crossing a reticulated python and a Burmese python. The babies are mules, hybrids, they are animals without identity. If that's what you're after, fine, but realize what you've got. Borneo x Sumatran crosses are not a "color phase," they are hybrid animals.