There is only one MAGI on a joint tax return and that includes the income of both spouses. That is accommodated by spouses filing jointly having a substantially higher deduction phaseout range that do single filers.
To have separate MAGIs, the spouses would have to file separately, but then the deduction phaseout would happen between $0 and $10,000 of MAGI, so filing separately would make no sense to do.
