So, you need a Sage for historical research...but you also want one who knows about the mating habits of purple dragons. I assume that that would be two different sages, right?
And what is the chance they will know about the field you are looking into? Is that included in the chance to find them?
I believe the chance listed is the chance to find each sage of a particular topic. so if there's a 50% chance, you make 1 roll for a historical research sage, and 1 roll for a purple dragon mating habits sage.
I'm not sure I want to know what you're involved in that requires an expert in those two subjects.
For what it's worth, the d30 Sandbox Companion (see RPGNow) has a sage generator which is kind of cool. It can generate major and secondary fields of study, specialties in the major field, and overall competence in all fields of study. If you decided there was a sage that knew about, say, dragon mating habits, you could then go and use the d30 Sandbox Companion to flesh them out.
(FWIW, it's a great little companion for all sorts of random generation in the OSR style of gaming. Best $10 I've spent on OSR gaming (outside of ACKS, of course ;) There's also the d30 DM Companion which is more dungeon-oriented (less up my alley, but still good)).