I haven't seen them on VMS's page, but two codominate traits can be expressed at the same time. Just look at the bumble bee ball pythons--those are pastel spiders, which are both codominate traits. I'm a little rusty on the terminology, but as long as two traits don't occupy the same allele, I believe they can co-exist. So yeah, I don't see why one couldn't produce "big macks". You could also think of it as recessive traits, and the animal being double het for the super form of both the mack snow and giant.
That is exactly what it is. Sean put a male mack with one of his super giant females (Probably albino) creating giant macks. "Big Mack" is just a catchier slogan. He got his giants from us in 2003, we were one of the first to purchase giants shortly after Ron released them in Aug. 02, but got bored with them and sold them to Sean @ VMS. Long story short I bought some giants from him last year, and did the same cross. Looking in the incubator a few minutes ago, there were two snows down there, and an albino mack from another group!! Love these guys!!