If you can't find out what it is, then enter zero. It won't matter
unless you live in a community property state. Income allocation is only
really required in community states. Community property states are: Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington, and Wisconsin.
If
you do live in a community property state, and there is an issue with
your filing, the IRS (and/or state) will notify you of any adjustments
(if needed) once your spouse has filed.
https://ttlc.intuit.com/replies/3301943