Sigil Captain

If, for some reason, Sigil Captain enters as a 1/1 creature (due to a pair of Engineered Plagues, for example), it will cause its own ability to trigger.

For that reason, two Sigil Captains don't work well together. When a 1/1 creature enters under your control, the abilities of both Sigil Captains will trigger. When the first resolves, two +1/+1 counters are put on the 1/1 creature. When the second resolves, it will do nothing because the creature that entered is now 3/3.

Sigil Captain's ability has an "intervening 'if' clause." That means (1) the ability won't trigger at all unless the creature that enters under your control is 1/1, and (2) the ability will do nothing unless that creature is still 1/1 at the time the ability resolves.