You will find a pattern here: this event is so powerful that you should play both copies every single game. If you feel this event is clogging your hand, remember that even for half its effect it’s still worth it! So while it’s tempting to go for the full two life and two damage, even half of it is good enough. This is especially true because it’s the only tool the deck has to kill units outside of the standard three attacks. 
Transfuse is particularly potent when combined with the champion Vesna Brana.