There's more to this.

You shouldn't arbitrarily allocate the income.  First, it depends on the type of tenancy they have.  Second, joint tenancy does not necessarily establish the right to income or how the income is shared.  These are determined by local law.

There may also be gift tax implications.  Although NRA's are not normally subject to gift tax, gift of US real property is one exception.  Things to consider are how the acquisition and other expenses such as subsequent improvements and pay down of debts are funded.  If the acquisition happened in the recent past, this may only be recognized down the line when the tenancy is terminated (e.g. by the sale of the property, which, of course, is then subject to FIRPTA).