question about green card

My hu*****band and i applyed the canada green card for backup.It was approved in Dec 7 2004.It is a good news for us.But we still want the USA green card.We need to move to Canada within next 4 years.Can somebody has the good ideas how can we do will keep two green card valid?Because if you no lognger live in USA half a year or live in canada 3 years within 5 years.Your green card will be voided.I appreciate your respond.