I’ve heard of two Christmas Novenas; the St. Andrew’s Novena that begins on November 30th and ends on December 24th (which I’m currently doing) and this second Christmas Novena that coincides with Las Posadas in Mexico and the “O Antiphons.”
As for the second question - as long as you take the time to properly pray your novena, I don’t see why you couldn’t pray two at one time. I prayed the Immaculate Conception Novena and the St. Andrew’s Novena together.
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