By signing Gerald Sensabaugh, the Cowboys aren't necessarily halfway to completing their starting lineup at safety.
The team definitely is, and should be, looking to sign another veteran. But they might not feel forced to sign someone, and probably won't overpay, because Sensabaugh can play either of the safety positions.
So, if the Cowboys really like what they've seen of Akwasi Owusu-Ansah or Barry Church in practice, Sensabaugh could fill the other position. Church is strictly a strong safety, and Owusu-Ansah is more a traditional centerfielder at free safety.
It's probably better than banking on both second-year players succeeding this season.
If the Cowboys didn't bring in another veteran, they could at least make an educated guess about which young guy is more ready to start.