Anyone ever set up a profit-share license?

Hi folks,

I've been selling a simple SaaS web app for a few years. I wrote the front-end, and my friend wrote the back-end. The front-end is hosted on my server, and interacts via an API with the back-end which is hosted on my friend's server.

We have an unwritten 50/50 revenue share agreement and each month 50% of the revenue goes to my company's account and 50% goes to my friend's account.

The transactions are handled through FastSpring's split-pay feature, so my friend's 50% goes straight to his account. It's not like it goes to my account and then I manually transfer it to him.

I'd like to put the agreement in writing for the sake of clean record-keeping and clear lines of liability/indemnity. For instance, if I make an error or omission (e.g. copyright infringement) on the front-end, I don't want my friend to be legally liable. And if he makes an error or omission on the back-end (e.g. his server is hacked and personal data is leaked) then I don't want me or my company to be legally liable.

Does anyone have a recommendation for what type of agreement I need? Would it be a license agreement, profit-share, joint-venture, or something else? I'm looking for the simplest, cheapest option that does the job. Thanks in advance for any advice!

P.S. I'm based in Australia in case that's relevant.