So far the Spider gene appears to act in a "Dominant" manner.
What this means is that there does not appear to be any visual(phenotypic) change between a heterzygous(one copy of the gene) and homozygous(two copies, also called a super in co-doms) individual.
There is a chance though that it could be a co-dominant mutation, whether it is visually different(color/pattern) or phenotypically different (think fatal, if the young die before reaching sexual maturity this is called a lethal trait). Who knows though, since not very many Het X Het breedings have been done except by Kevin (N.E.R.D.) at the beginning of the project.
Hope this helps,
Dan
p.s. as a side note, there have been rumors of normal looking spiders producing 100% spider litters when bred to a normal, but so far not a single spider has been proven to be homozygous.