Two-headed puzzle

Following advice from French friends, this week has given the opportunity to visit a few more new places in Paris.

One asked me "Why would an Englishman want to see Napoleon's tomb?  To spit on it?" and I replied that the past is a foreign country and that I didn't need to go there to gloat any more than I boast that I went to the same school as William Wilberforce (who was instrumental in the abolition of slavery).

Two-headed

You can find photos of the tomb anywhere, but this carved head provided a moment of surprise walking back out of the Hotel des Invalides.

What is the point of representing two heads in one like this?

Any ideas?  It must mean something.