Deborski:

Apparently this was originally a 1923 poem by Михаил Кузмин. The original had an extra verse not included in your version of the song:

Золото в потолке зодиаком,
Поминальные по полу фиалки,
Двустороннее зеркало круглеет...
Ты и я, ты и я - вместе -
My attempt at a translation:

The gold on the ceiling forms a circle like the zodiac.
The funeral violets form a circle on the floor.
The two-sided mirror looms circular.
You and I, you and I, together.

In other words, I think that the singular verb круглеет ("to become round" or "to have a round appearance") is meant to refer back to the three nouns золото, зеркало, and фиалки, even though the last one is plural. But I could be wrong. Also, based on Googling, I think that двустороннее зеркало means one of those mirrors that reflects differently on each side (like a cosmetics mirror), as opposed to a semi-transparent mirror used in the interview rooms at police stations.