There's still very little details about LittleBigPlanet for the PSP, we aren't exactly sure what it will contain. And while the latest screens only show one SackBoy, it's be pretty much video game suicide to not include co-op in the next incarnation of this title.
So far all we have to go on is this quote from the official release:
"Imagine a whole world where you could play, explore and create whatever you like - and imagine that you could take that world with you wherever you go."
I'm imagining it, and I'm imagining it with co-op.