If you are using a two part epoxy kit like the G2 then you have limited time before the paint starts to harden! I actually prefer to jack the car up on all four corners, My car is sitting on four stands as we speak and has been there for a couple of months waiting for my new engine! Jack up one side of the front and put a jack stand under the reinforced jack stand flange then repeat for the other side and now the two front wheels are off the ground and on jack stands.
Now take your floor jack and lift the rear fron the bottom of the diff but make sure your car doesn't "creep" rearward as you are lifting, place the two rear stands at the proper points and reverse that order when you are done.

 

No it is not absolutely necessary.....

Yes it is possible to do one at a time.....

If your not comfortable with the car on stands or don't have the right equipment, then just do one at a time.

 

I think it's safe to put it on jack stands. I've had mine up on them for 2 weeks while waiting on my wheels to get back from the powdercoating place.

 

I

 

put them on jack stands and paint them all, if you use the G2 paint, it drys up quickly, so put a coat on one paint, move to the next, continue on, and apply few multiple coats until you like it

 

^^ I agree that stands are recommended for safety reasons. I was answering the original question of is it "absolutely" necessary and is it "possible" to do one at a time.