I think you need to have things clearly laid out in writing before hand, so that if they are friends you can keep on being friends.

Clear definition of the duties for each person.

Clear fiscal responsibilities for each

Clearly defined goal of the business

Independent 3rd party arbitrator for when there is a dispute so that it doesn't escalate into name calling

Where each has their own strengths that are not in conflict with the other's strengths and/or weaknesses.

Mutual respect

Mutual work ethics

being cute is an option

just my 2 cents
Ian