It is my understanding that the rater and Senior Rater can be the same person but that the reviewer must be another person. If there is not a military person in the rating chain an enlisted evaluation should be reviewed by a senior NCO.
EXTRACT AR 623-3
2-21f. Supervisor as both rater and senior rater
On NCOERs, a rater may act as both the rater and senior rater when the rater is a general officer or a civilian employee with SES rank and precedence (para 2–8 b(3)). A promotable COL working in a BG position may also serve as both the rater and senior rate