The turnover threshold is calculated per person for property income, but per business for self-employed income. If you are a sole trader you must aggregate your self-employment income with your property income to check if you exceed the £10,000 limit. But you do not aggregate partnership income.
Thus, a partnership with gross turnover of £10,000 or more must comply with MTD ITSA. However, it is the partnership that must make the MTD ITSA submissions, not the individual partners.
When looking at property, married couples are treated as receiving income from property equally unless they have submitted a form 17 to HMRC.
If either of them is a sole trader they must add their self-employed income to their rental income to decide if they are subject to MTD.
However, if they are in a partnership, their respective profit shares are not added to their rental income.
In contrast, where a property is jointly owned by individuals who are not married or in a civil partnership, those owners are free to allocate the property income between them as they see fit.