Just to let you know, there has been no research done on weather or not a homozygous spider is lethal.
The reason people don't usually pair spider to spider is because there is no visible super form and its time better spent breeding them into combos than each other.
Personally, i think the homozygous spider can exist (just like the homozygous pinstripe), it just needs to be proved out by a ton of breedings people usually don't feel like doing.