When one of you goes back to studying

This is the rare income change that can be planned properly, and the households that do it well treat it as a project with a start, a rule and an end date rather than as a permanent new arrangement. The split moves to proportional for the duration, the duration is written down, and both people know what happens when it finishes.

Decide whether it is an investment or a choice

Both are legitimate and they lead to different arrangements. A course that raises one person's earnings is often treated as a household investment, with the other partner carrying more for a period and no expectation of repayment. A course somebody wants to do for its own sake is more often carried by that person out of their own money.

The trouble comes from never deciding which it is, because each person can then hold a different answer for two years without either of them saying so.

Fees are not the number that matters

The tuition is visible and planned. What catches households out is the lost income — the hours not worked for eighteen months — which is usually the larger figure and rarely the one in the spreadsheet.

Name the end

A stipend that becomes a salary in June should not still be paying a student's share in September. Agreeing the date the split returns to normal, at the start, means nobody has to raise it — and raising it is the awkward part.