This is usually the case even if you are not an adjunct -- which is why so many people are interested in team-teaching and so few actually do it. If you share a course with someone, you each do a full course's worth of work but only get half the credit.
We can do it here and both get full pay if we double the class size.
For a graduate course, when we discovered that grad classes can not be cross-listed (and the two of us came not only from different departments but from different schools within the university), we successfully managed for several years to each teach a course in our own graduate program . . . both just happened to meet at the same hour, so after the first meeting they could be combined in the same room. This worked only because graduate classes in one of the departments are always in late afternoon / early evening (so finding an available classroom was no problem, and no official person needed to know about it). We wouldn't have tried, though, had we not both been tenured full profs.
And I would suspect that an adjunct at most places would have trouble proposing and getting approval for a new course, let alone getting approval for and paid full salary for a team-taught course.