If the transfer results in a change in the basis of the traditional IRA of either spouse, both spouses must file Form 8606 and follow the directions in the instructions for that form.
That's the problem. The instructions for Form 8606 only explain how to REPORT the basis allocation between the parties. It does not explain how the basis allocation must be determined.
For a Roth IRA, the equivalent question would be whether the Roth could be partitioned in any manner other than pro rata, eg having the petition assign all the earnings to one party. That's even more unlikely to fly with the IRS since there would be no 8606 tracking until distributions were taken.