CAIS and home office expenses

On the CAIS information form and the T1163, why are the expenses for use of the residence for business purposes prorated between the partners?

Form T1163 does not provide any alternative for such a situation.

To work around the problem, the CRA recommends that you enter the total amount of expenses for use of the residence for business purposes on a second CAIS information form and not report them at all on the first form.