Two Earths Are Better Than None

The alien ships appearing across Earth come seeking help, not conquest. The Peria have crossed from an alternative reality, part of a Duality caused by the Schism. The Duality is collapsing. Unless all races work together, everyone could die. Mason Ross discovers he's one of the select few whose consciousness can cross realities - just what the Peria seek. As his marriage collapses on both Earths, helping the Peria sounds like the fresh start Mason needs, especially as they're offering a fortune in exchange. Mason joins a small group aboard a Peria vessel, including Beth Borden, who takes an instant dislike to him.
It saves time. During their months of training, they come to understand the full scale of the threat spreading across the galaxy. Not even their Peria captain, Hobson, understands what's going on. One thing is clear: they can't solve it alone. It will take an alliance with other aliens to counter the actual threat, armed with little more than Beth's sarcasm.