"Send invoices to US clients in USD and Canadian clients in CAD"
When adding a new Customer, you have option to pick currency (at the very top). You can't change currency later. QBO will pick invoice currency based on this selection.
"Track time against US and Canadian clients and need to add those time entries to USD and CAD invoice"
Not possible in QBO. You can probably find a third-party solution for that.
Before you look at the functionalities make sure you check that the integration is available for multi-currency. I know some app such as Function Point has all the cool features but when it comes to integration, they suggest running two instances of QBO to make it work.
