It's not really the same question that I asked, since those licenses don't do the same thing for competing interests.Sure; no one suggested that you needed to use the d20 STL to use the SRD (and you didn't; lots of products used the SRD without using the d20 STL), though I do think that the d20 STL wasn't something that could be used (in any meaningful way) without also using the OGL. But your question was if a book could be released under two licenses simultaneously, and the slew of OGL/d20 STL products that we got make it very clear that such a thing is entirely possible.