Xanth by Two: Demons Don't Dream and Harpy Thyme

Demons Don?t Dream begins a Xanth sequence as young adventures play for the highest stakes of all: the future of Xanth--and of Earth as well!

Dug thought that the disk he got from his friend was just a fantasy game.  He didn't particularly like fantasy games, but he'd made a bet with Ed, and a bet's a bet.  So he loaded the game and?

Within moments he had left Mundania and was standing in a forest glade in Xanth, with his Companion in Adventure Nada Naga -- a beautiful Princess who just happened to be a shape-changing serpent.  For a long time Dug thought that this was just an incredibly convincing virtual world.  Then he began to believe in Magic.  Which was a good thing, because the prize in this Game is the Power of Magic in Xanth.

In the next installment, Harpy Thyme, Gloha must brave a demoness and snow dragons, malevolent clouds and musical volcanoes, on a wild odyssey that takes her not only across the length and breadth of Xanth, but on journeys through Time, Space, and the perilous realms of Madness and Mundanity.

Two Xanth novels in a single volume:  Twice the magic, twice the adventure ? Xanth By Two.