This. And any verbal agreement or understanding you have with your partner (Or partners), it didn't happen if it is not down on paper in written form, clearly spelling out whatever the agreement is, signed and dated by both parties.
Everything changes in time, people change, business needs change, get it spelled out who is responsible exactly for what and how. And have a decent attorney review any addendum's or alterations to the original agreement.
Just a suggestion based on lot of experience. :-)