Is it possible to start the application even before getting a job in US?

First of all, you can petition for your parents even while you are still overseas. Secondly, you can petition both parents at the same time or at different times since they each needs a petition filed on their behalf. They should be able to visit in the US while the process is pending since as Canadian citizens they are visa exempt.

You can file for your parents, for both at the same time or one at a time (your choice). I recommend that they remain in Canada from the moment you file and until their immigrant visa is issued. They could try to come and visit you while their immigrant visa is pending, but they could be denied entry if the officer at the border learns that an immigrant visa is pending for them. This is because, when crossing the border for a visit, it is implied that they have the intent to return home. But when an immigrant visa petition is filed for them, the intent is for them to immigrate to the U.S. Thus, there is a conflict. If they drive across the border and no one stops them, they should be fine. If they are asked a question, they cannot lie. If they are turned around as a result of the pending immigrant visa petition, they need to be prepared to turn around. The same scenario goes with flying to the U.S.: A pending immigrant petition may prevent them from entering the US for a temporary visit. You can petition for them while you are still overseas, but you need to have a US address where you reside and where your parents will reside. Also, you need to be able to sponsor your parents, so a tax return is required. We have many clients who are US citizens working overseas and as a result, they have issues with their US tax returns. You should make sure that you are in compliance and up to speed with the US taxes even though you work overseas; this is very important.