If both people have the same type (1 or 2) genitally, then they are safe to have sex without concern. It is true that there is a small possibility that a person could acquire more than one strain, but research indicates that even if this does happen, there is no clinical significance to that event.
The person who has HSV 2 genitally is highly unlikely to get HSV 1 genitally. The research suggests the chances are about 1 in 55,000. However, the person who has HSV 1 genitally can still get HSV 2 genitally.