What does it mean when a radiologist say both kidneys demonstrate a symmetric enhancement?

See answer. It would indicate that both of your kidneys are functioning alike, presumably normally. Kidneys normally enhance after contrast material, through a vascular and then an excretory phase. This need to be placed into context with your entire imaging examination but likely things are fine.
Kidneys. This simply put is term to describe the absence of differences between the kidneys. Enhancement usually refers to how the dye looks in the kidney or how the tissue density looks . It by no means on its own indicates a problem.