What is unique or rare about the VM Zodiac signs?

[MarcoP]

The drawing style, dress, faces and even the trees in the background all look period appropriate. So even if they were not original, then I'd say the design certainly is. 

If this is a man and a woman, then the pose is what I'd expect of a marriage or betrothal contract, whereby they hold hands while the man offers an object to the woman.

Hi Koen,
this French web page confirms what you say:
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Of the zodiac signs, only Taurus and Capricorn were replaced in the XVI Century.

The two figures appear to be males: the women in Virgo and Libra have longer dresses that cover their feet.
The Gemini illustration is interpreted as the two men exchanging two objects with both hands, but (like you) I only see an exchange in the upper hands. It could be a leaf or a small plant, as the French web page says.