Hi
I don't see why you couldn't keep both. The standard for keeping the US green card has for many years been that you have to come to the US every 6 months and since most places in Canada are not too far from the border, that is very doable. To keep the Canadian PR card though, you have to actually live in Canada for at least 2 years out of every 5 so if you did decide to move back to the US, you can only live there for 3 years, then you would have to come back for 2 if you wanted to keep your Canadian PR.
I suggest you read the following:
http://www.hooyou.com/i-485/MaintainlegalPRS.htmlPMM