Do you have a better idea? I'm open to suggestions.
How about we denote dual-type temperaments by the temperament-based attribute shared between the IE and IM type.

For example, an Ixxj-Exxp would be called 'static-bound'. The Exxp-Ixxj would carry this label also, which makes sense, as the two temperaments have much in common. A group of people for whom it is easiest to take a fresh look at problems and to interface directly with the subject material.
Exxj-Ixxj would be a 'rational-bound' type. A safeguard of human knowledge, who sees how the established views of their times tie in with timeless principles.
Likewise Ixxp-Ixxj would be 'field-bound' or 'introvert-bound'. A person who evaluates different models of understanding by contrasting them against eachother.

These descriptions are probably biased and generally not very good, but they reflect how the temperaments appear to me.

I find it easiest to tell a person's dual-type temperament by looking at their most outstanding temperament based quality.