Halfling Agility is a heroic feat that gives the triggering attack a -2 penalty on its reroll. Nimble Dodge is paragon and similar to the first; It has a -5 penalty and the reroll cannot crit.
So could I use both and get a total -7 attack penalty?
Halfling Agility states:
Nimble dodge states:
They are penalties that come from different sources and are untyped, therefore they stack. This is fair because they're both buffing a singular encounter power and could therefore be considered a waste of feats. I recommend taking simply nimble dodge.