I decided to ask a question, and make it simple and for both of them. What I came up with was "So what would result if the two of you wrote something together?"
See, Hobson does turn-of-the-19th/early 20th century science fiction with a steampunk flavor; at this event she'd read from an alternate history novel of hers where Nikola Tesla was far more successful than Thomas Edison, and has an impressive factory/compound in 1910 Detroit. McGuire writes about zombies, urban fantasy, pandemics ("Airborne rabies: when that happens, that's when I shoot myself"), and the many ways in which humanity could end the world. When I asked, they found common ground. McGuire talked about her problems with most steampunk, summed up in this: steampunk wouldn't be all brown. "These were the Victorians," she explained. "They'd just discovered neon." So steampunk should be more colorful, she said -- no, exhorted, enthusiastically, describing garish colors. Hobson said "THAT'S why we need people like you writing steampunk!"
Ah, a good response at a fun event. I should use that for other authors when they're doing joint appearances. And afterwards, I was able to talk to McGuire briefly and thank her for her blog, which she remains very serious and enthusiastic about. (She said she's been on LJ in one way or another since "the beginning," and has a fanfic blog elsewhere on it.)
One more quote from McGuire tonight: "'Duology' and 'trilogy' are two words we misuse horribly... The only legitimate time you can write a cliffhanger in a series is the first book of a duology, or Book 2 of a trilogy. Otherwise you're doing it wrong, you make me cranky, and I own a machete."