Text (AR+errata): 3/3. ; Whenever a creature enters the battlefield under your control, if that creature is 1/1, put two +1/+1 counters on it. [Oracle 2009/07/08]
Because of the "if" clause, the ability won't trigger unless the creature is 1/1 when the creature comes under your control, and the ability will be countered if the creature is not still 1/1 at the start of resolution. [Alara Reborn FAQ 2009/04/30]
Having two Sigil Captains will not give an extra effect because the first one will give the creature two +1/+1 counters, so the second one's ability will be countered. [Alara Reborn FAQ 2009/04/30]
Any counters the creature comes into play with, and any continuous effects that alter power/toughness are counted when checking if the creature is 1/1. [Alara Reborn FAQ 2009/04/30]
