Oops! My bad. You do NOT have to be a limited company, according to the Greenlight FAQ. You can be a partnership, or sole trader. They will not split payments though, and REALLY, REALLY recommend being a limited company.
Seriously, if you put something out there without limited liability, and get sued, you could lose everything. Do NOT go into business without limited liability.
Frankly, if you are this worried, then you do not know the other person well enough to go into business with them.