It depends on how the business is structured.
If it is a separate company, the the revenues will be assigned according to your respective shareholdings.
If it's a partnership and you have told HMRC that you are self-employed (or partially self-employed), then you can decide how much time, effort, experience and resource each of you puts into the business, and assign the earnings according to that division.
On such a small business, I suspect (though you should seek professional advice) that you can just decide to assign the earnings in the way that is most tax-efficient, provided you can somehow justify the split you choose according to how much time, effort and experience each of you contributes.
Good luck!