Taking lamictal 100mg for bipolar and tegretol 600 mg--am I even getting a therapeutic dose being that tegretol breaks down lamictal by 50%?

Interactions. The stated interaction is accurate. Regarding therapeutic dose if desired benefit is achieved these answer is yes. If symptoms are persisting then consideration can be givening for testing for a lamotragine blood level.
Maybe. You could get a tegretol level to find out. Clinically, you can feel if your moods are more level even if the blood levels of the anticonvulsant is not therapeutic.