Spiders are co-dom, so you can get spider off spring from just one spider-gened parent. Axanthics are recessive, and you need the parents to BOTH carry the axanthic gene in order for the trait to show. ("het-axanthic" means the animals carries the gene, but looks normal)
So...to get a spider/axanthic snake:
1. Spider x axanthic ...then breed the baby spiders back to their axanthic parent.
2. Buy a spider that is already het-axanthic and mate it to another axanthic or het-axanthic. In this case you could get the combo in the first clutch...but you could also miss the odds. There's never any guarantees.