Question regarding stay USA and Canada combine

[tech2012]

Hi

I have question regarding USA/Cananda stay period for on visitor visa.
Actually my parents want to visit USA and canada. They can stay in USA for 6 month on visitor visa.
But My parents will go to Canada for 1- 2 month in between USA stay
So How many months time they can stay in USA ?

They will come to USA on Feb -2015 and stay here till May -2015
After they will go to Canada for 2 month – Jun and july -2015.
And come back to USA on Aug-2015. After Aug-2015 onwards how many months they can stay in USA ?

Please let me know ASAP.

Thanks