In the absence of a partnership proper, (whereby variable profit shares are very common), the co-owners can nevertheless rebut the presumption that income will be shared according to beneficial ownership.
This from PIM1030:
"Jointly owned property: no partnership
Where there is no partnership, the share of any profit or loss arising from jointly owned property will normally be the same as the share owned in the property being let. But joint owners can agree a different division of profits and losses and so occasionally the share of the profits or losses will be different from the share in the property. The share for tax purposes must be the same as the share actually agreed.
However, where the joint owners are husband and wife, or civil partners, profits and losses are treated as arising to them in equal shares unless:
both entitlement to the income and the property are in unequal shares, and
both spouses, or civil partners, must inform HMRC that their share of profits and losses is to match the share each holds in the property."
Of course such arrangements may amount to a settlement. But the settlement anti-avoidance provisions do NOT act to revert the income to the Settlor UNLESS the Settlor (or his or her spouse, etc.) still benefits from that diverted income - either presumed in law (broadly) as with settlements on spouses / civil partners / minor children, or as a matter of demonstrable fact otherwise.
So, the direct answer is: "however you darn well please".