His theory was along the lines of this: the first book took you years to write, and in finishing it, getting an agent, getting published, etc., you now have to produce the second book on a much tighter timeline. This means the quality will suffer in the second book, and with the lower quality, the sales will be low, and the publishers will have reason to drop you.
His basis for these comments were in reaction to a former member's two books, that came out within a year of each other. He spoke of the second one as if it was a rush job, and thus of lower quality. But then he admitted he hadn't actually read either one, so he didn't know about its quality, and I'm not sure what his point was. Though he strikes me as a person who looks at things in strange absolutes; a novel takes a certain amount of time to write, just like a chicken takes a certain amount of time to roast, and you can't change that without sabotaging the quality.
I, personally, think this is all a load of crap.
However, I think his point about two-book deals isn't entirely wrong, but he's wrong about it being a bad thing. Namely, it's a very good tool for publishers to tell if they've got a potential career writer on their hands, or if they have someone who's only got one great book in them. Are you the kind of writer who can work under a deadline, craft what they need with skill and competence, and thus be someone whom they should invest further in? That's something the publishers need to know.
I'd like to think that I'm that kind of writer.
So, my only problem with two-book deals is I don't have one yet.