It would seem that Janus as the force of duality, polarity, and universal passage would also be connected to Sol Invictus since they were both celebrated at the start of the new year for having conquered death and eternally governing the Celestial cycles. Will there be discussion on the connect with Sol Invictus with the new book because of their seemingly merged characters.
No, Janus is discussed in the book in connection to Lucifer as the Lord of Thaumiel.
I have come to know Sol Invictus as an aspect of Lucifer so I would say it is indeed a correct connection.
Heard it from the Serpents Key. Which is how I have come to know that. Forgot to mention.