I have connected an LCD monitor to my MacBook Pro 13". Can I use the trackpad for the MacBook Pro screen and a USB mouse for the larger monitor?
Let's phrase it like this: No.
Simple reason: You still only have one computer.
Longer reason: When you connect two displays to a Mac, it'll either mirror them (or "clone", to be precise), or extend your desktop onto the external display. However, you still only get one mouse pointer. The system doesn't support more than one at a time, and it'll take both your trackpad and an external mouse as an input device for this mouse pointer.
If you want two entirely separated systems, you can run OS X Lion in a virtual machine, and let this VM run in fullscreen on the external display. That still will only let you use one mouse pointer though, which is exclusively used for either your host system or your guest system.
Use virtualization, where you can run 2 Operating Systems side-by-side, I use virtualbox which an opensource software. However, for best expereience you will need more RAM, roughly about 6-8 gigs.