Except for the fact in cases where effects from maneuvers and such carry over into other rounds it is specified and with Aim it isn't. Read Guarded Stance, in AoR, then read Aim. No mention of carry over with Aim which means you can't. Plus, why would you even want to. Two shots with a single Aim are vastly more productive than one shot with 2 Aims.
I am afraid I don't have AoR so i can't read it... does it specify GS can be performed in consecutive rounds?
Anyway, wanting (or not) to do it depends totally on the player. Maybe he prefers to be "extra" sure to hit and bets for a more prepared shot.