In Weston Ochse and David Wellington's "The Lessons of Room 19," vampire hunter and ex-con Laura Caxton is blackmailed into helping Seal Team 666 member Jack Walker rid himself of his dependence on a “ghostskin” that bears the spirit of his dead girlfriend. Ochse’s Seal Team 666 series is in the CU through references in Jonathan Maberry’s Joe Ledger books, so this story brings in Laura Caxton, the protagonist of a series of novels by Wellington.
In Larry Correia and Jonathan Maberry's "Weaponized Hell," the aforementioned Joe Ledger, of the Department of Military Sciences, and Agent Franks of the Monster Control Bureau battle terrorists who are summoning demons. Ledger and Agent Franks (from Correia’s Monster Hunter International series) are both in the CU.
These crossovers are only two of hundreds covered in my book Crossovers Expanded: A Secret Chronology of the World Volume 3, which will be published by Meteor House! All three volumes are AUTHORIZED companions to Win Scott Eckert's Crossovers: A Secret Chronology of the World Volumes 1 and 2!