Moving data from one table to another and making a portal record disappear in one portal to reappear in another are two very different operations.
Except for certain back up operations, moving records from one table to another isn't often the best design.
Making a related record in a portal "move" from one portal to another can be done in different ways depending on what you want to accomplish. You can use filtered portals in FileMaker 11 to make this happen in some cases and in all versions of FileMaker, you can manipulate values used in the relationship to drop the record out of one portal and bring it up in another.
Are both portals related to the same parent record?
Are you using FileMaker 11?