Loving the Texas Stranger by Mary Connealy

An assassin takes a shot and hits two people with one bullet.
Natalie Brewster, a deep undercover agent for a group that doesn’t officially exist, saw the gunman aiming. He definitely waited, lined up the shot, and tried to kill both of them. Nat and Case Garrison hit the ground bleeding. They both come up shooting and kill the assassin—but that just makes him mad.
Now Case and Nat, who have never before met, have to figure out what they have in common to make them prey for a killer.
And they have to do it while running for their lives because, despite the two of them being highly skilled at keeping a low profile, the killer keeps finding them. They are running at break-neck speed, hiding with all the skill of long-time undercover agents, and falling in love with a passion that might be part of what brought a killer down on them.