When a donor/prospect is an invited guest another donor, you cannot add the guest's spouse. If the donor has been added as a guest under another donor, then you should be able to add the spouse as a guest as well, especially if the organization uses family records. Currently, you have to change the invited guest to a "host" and then add their spouse, but you cannot link the gift to their registration because they didn't pay. They are not a host, they are a guest of another donor, as is their spouse. TLDR: If a constituent is a guest of another constituent, include a way to add the invited guest's spouse if they in the relationship records.