I think the general opinion is that two plants in the same soil will tend to conflict with each other. For a PC grow, one plant is seriously more than enough.
If you don't want to sound discouraging... Then I suggest you don't discourage people in the first place.
Smatz, ignore the naysayers. They love to say 'more lights, bigger box, can't be done'. I'm growing in a box almost half the size of yours, and on my first grow (and I am NO kinda expert) I pulled just under an oz, that's off three weakass 23w CFLs and some lazy lst work. Most people seem to say the Autos are pretty short too, so I can't see why they wouldn't work a treat for a pc grow. Personally, I'd say spend out and get some regular feminised seeds, and flip them when you wanna flip them. You've got so much more control that way. My advice is to just get things rolling, you'll get a lot more help once people start seeing you make some progress. Get pics of everything uploaded, the setup, the seeds, just keep things fresh. There's a tight little community on this corner of the site that are all doing amazing things inside small pc cases. Start up a grow thread, get your seeds, get them in the dirt and learn as you go. It's the only way to really find out how you like to grow.
mewk.