Rutilius was not alone in using the singular Symplegas, but he is keeping odd company, according to Lewis and Short s.v. Symplegades:

I. Two rocky islands in the Euxine that, according to the fable, floated about dashing against and rebounding from each other, until at length they became fixed on the passage of the Argo between them, Mel. 2, 7, 11; Plin. 4, 13, 27, § 92; 6, 12, 13, § 32; Ov. M. 15, 338; Hyg. Fab. 19.—In sing. Symplegas, Val. Fl. 4, 221; Luc. 2, 718; gen.Symplegados, Val. Fl. 5, 300; acc. Symplegada, Claud. in Eutr. 2, 30.—

II. Transf.: symplēgas , ădis, f.; as an appellative, a joining together, cohesion: “praebente algam densi symplegade limi,”  Rutil. Itin. 1, 461.—Of the buttocks, Mart. 11, 99, 5; Aus. Epigr. 108, 8.

Martial 11.99:

De cathedra quotiens surgis—iam saepe notavi—
Pedicant miserae, Lesbia, te tunicae. 
Quas cum conata es dextra, conata sinistra
Vellere, cum lacrimis eximis et gemitu:
5Sic constringuntur gemina Symplegade culi
Et nimias intrant cyaneasque natis.
Emendare cupis vitium deforme? docebo:
Lesbia, nec surgas censeo, nec sedeas.

A translation can be found here