Let's say I have US and Canada tourist visas. I can stay in Canada for 6 months and must then leave the country. Could I just go to the US for a few days and come back to Canada, as this would be a new entry and I could stay In Canada for 6 more months?
The limits you state are a maximum, not an entitlement. A border officer has the power to restrict the length of time you can stay, or deny you entry, even with a valid visa.
What will probably happen if you try this is that when you try to enter the second time the border guard will ask why you are coming to Canada so soon. Unless you have an exceptionally good reason he will deny you entry, while pointing out that trying to live in Canada on a visitor visa is illegal. Subsequent entries are then going to be even more difficult.
If you have no reason to go to the US other than extending your status, there is also a process to apply to extend your status from within Canada.