Of course the former is very common with Auto Scaling Groups [1] [2]. Then you can use round robin or session sticky routing algorithms in the load balancers.
(Apologies if I'm totally off-base for what you were asking.)
1: http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide...
2: http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide...