Opus Gemini I by Romanike

Around 70 B.C., a tradeship heading for Rome foundered off the coast of Antikythera. On board was the most sophisticated device known to the classical world:
The Antikythera Mechanism - a set of gears that might foretell the ways of the stars.
In the sea, the Antikythera Mechanism escaped the purposeful destruction that hit its siblings a hundred years later. Save for one: The OPUS GEMINI. This last of the Opera is held by Adrianus and Valeriana, two quarrelsome housemates at the rim of the Roman world.

There comes a time of crisis, of warfare. And the ancient power that once ruled the Opus rises again. The agents of the lost realm of Commagene set forth to bring the OPUS GEMINI:
Pernica the Swift, also known as the Polan Panthercat, who is seeking her life and her doom.
And Sedigitus the Sixfinger, a misshapen warrior, dedicated to his mission at whatever cost.
But there is another, a new force gathering. The mysterious Headhunters are out, and they take more than just the heads of people they are interested in. Adrianus and Valeriana find themselves pursued by two enemies at once.

Each of them determined to get the Codebook of the Cosmos. And the man who knows to use it.