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
2–19. Review of noncommissioned officer evaluation reports
a. Every NCOER should be reviewed by the 1SG, SGM, or CSM to ensure accountability of Soldiers’ evaluation reports and to oversee the performance of junior NCOs. This is in addition to the review by the designated reviewer in accordance with paragraph 2–16, if applicable.
b. The reviewer will—
(1) Ensure that the proper rater and senior rater complete the report.
(2) Examine the evaluations rendered by the rater and senior rater to ensure they are clear, consistent, and just in accordance with known facts. Special care will be taken to ensure the specific bullet comments support the appropriate “Excellence” or “Success” or “Needs Improvement” ratings in part IV, blocks b through f of DA Form 2166–8 (seeDA Pam 623–3 for definitions).