TL;DR: There is no policy on this and we plan to handle any events on a per-case basis.
The best policies are based on studying and understanding situations fully so that the policy reflects actual behaviors and needs rather than our preconceptions of possible behaviors and needs. While there are situations where preemptively creating policy is necessary, it should be based on the risks of the situation and how likely the policy will be needed.
In this situation, we've considered the risks - there are some, such as a candidate potentially circumventing system-imposed restrictions if one of the two accounts has been recently suspended, but they aren't insurmountable. Other risks involve the moderator Team but can also be addressed as needed.
We've also considered the need - there is none. In the 15 years of Stack Overflow, we're not aware of any cases where a sock puppet account actually nominated in an election. While we're aware of cases where a sock has expressed interest, even those are minimal - possibly only a single case.
As such, we see no benefit to creating policy at this time. We will address any occasions of this occurring that are raised to our attention or that we discover on a case-by-case basis as we feel that we don't have enough information to guide a policy or reason to create one at this time.
If users find themselves in a situation where they're concerned that a sock puppet is being used to run in an election, they are free to flag the nomination and explain their concerns, which will be shared with the Community Manager leading the election directly.