Text (SH+errata): Choose two target creatures controlled by the same player. Exile one of those creatures and put two +1/+1 counters on the other. [Oracle 2009/07/08]
You choose the two target creatures during announcement, and you choose which gets the counters and which is to get removed from the game during announcement as well. [D'Angelo 1999/06/20]
A creature will not be affected if it changes controllers before this spell resolves. This is because being controlled by the chosen player is a targeting restriction. [WotC Rules Team 1998/03/31]
If one of the targets is not valid on resolution, it still affects the other target normally. [Duelist Magazine #25, Page 30]
