It can definietely be done, but it will take a lot of discipline, and planning. The really good thing is that when one novel gets a little hard (writers block and the like) you always have something there to wrote for the other novel. It takes a lot of will power to keep going with both especially when mentally exhausted.
I usually have two or more novels going at one time. This month it's three, another in the planning stage and I cranked out four short stories. Can it be done? Sure. Does it hurt? Not really, they are different genres with different styles so it's easy to keep them on track. If they were the same genre and style, it would be a challenge to keep them unique. If they are a series, then the challenge diminishes.
Does thinking about future novels count? Then I'm working on about 15 at once. The cacophony of ideas made it hard to stick to just one.
Doing two would be doubly hard. I could do it if I quit my full-time job and focused solely on writing. And if I had an office and didn't write on the floor of the living room. And if I had an IV and a drip bag of black coffee pumping directly into my veins. And if the fishing was really terrible, since I'd be tempted to take the fly rod to the river.
I am always working on multiple projects. But then, I'm a person who has multi-tasked all of my life. Right now I'm working on the third Silver Sisters Mystery, VANISHING ACT IN VEGAS, the final draft of a romantic suspense novel, THE DEVIL'S DUE, which will be released as book two of a set in mid to late 2010 by L&L Dreamspell under the pen name Arliss Adams. While working on book one, I was also writing the Silver Sisters Mystery SEVEN DEADLY SAMOVARS with my sister Phyllice Bradner, to be released in September 2009.
In addition to VANISHING ACT, I'm working on a humorous word book with a co-author, publish the newsletter ON THE PROWL for Sisters in Crime/Southern Nevada, and completed two short stories which will be included in anthologies in 2010. So it is possible. I don't know if I've ever had just one project in work. Sometimes it gets a little difficult, however.
I'll admit, I found it hard to swing back and forth between the comical crime capers of the Silver Sisters to the deeply emotional plight of Jen Connor in A DREAM LOST and THE DEVIL'S DUE. It's much easier to write in the same genre when balancing two or more manuscripts.