It looks to the me like the hull of an upside down boat that these two are sitting upon, and I'd guess they are meant to represent the twins of Gemini, Castor and Pollux, who were also mythologically associated with St. Elmo's fire int heir role as the protectors of sailors. Janus was associated with sailing too, and with backs facing and arms overlapping, it looks as if these two could also be one! In any case, what a gem of a little watercolor, and I believe quite an old one, I'd guess cut from an early 19th century manuscript. 

Sold in the antique wooden frame I found it in, in which it appears to have lived for quite a long time and which suits in perfectly I think. Framed: 5 3/8" x 4 3/4" x 9/16. Very good condition with super saturated color; a few old creases along the blue band of the boat.