You can transfer to another person's frequent flyer account only if that person holds a supplementary card on your American Express card account. There is not an automatic exception for spouses.

Possible exceptions:

If another person's frequent flyer account was already linked to your Membership Rewards account at the time the system was changed to enforce this rule it will still work. The check is done when linking, rather than when transferring.

Some have succeeded when requesting transfers by telephone, but this depends on the agent's discretion or lack of attention to detail.