USA EB2 priority date is now Jan 2010 ,faster than Canada PR .so many people who applied usa green card and Canada PR TOGETHER ,might be they will get green card before Canada PR ,
Legally you can not hold 2 countries PR at one time, once you get canadian PR than you can withdraw your US GC applicantion. if you got your USA GC before Canadian PR than you can withdraw you canada PR application or do not land to finish your landing procedure after getting PR visa & inform your VO to cancel your visa and PR.
but remember once you get EAD that is not your PR exactly the greeen card means (There is a big difference in EAD and GC. EAD is always temporary in nature. You will be always dependend on USCIS.). but once you get your USA GC than you must have to cancel one of the PR either canadian PR or US GC. choice is yours.