I think it would depend on the individual strengths of the different weighted materials. A more specific question (and perhaps more relevant) is, how consistent is the strength of ripstop by weight? That is, does the strength increase linearly with denier (which is what determines the weight)?
Another interesting question, pertaining to your double-layering plans, is whether or not two layers of ripstop is stronger than the sum of the strengths of each layer individually. I think probably not - a bridge or a laminate beam is stronger than the sum of its parts, but that's because the pieces have lateral bracing between them. This isn't the case with a "standard" two-layer hammock. The layers are only connected at the edges, which isn't where most of the stress lies anyway.
"Physics is the only true science. All else is stamp collecting." - J. J. Thompson