Complex sentence: Their supposition that she did not have Multiple Personality Disorder was, however, later supported by Dr. Herbert Spiegel, Sr. Spiegel saw Mason for several sessions while Wilbur was on vacation.
Split into simpler sentences: Their supposition that she did not have Multiple Personality Disorder was later supported by Dr. Herbert Spiegel. Dr. Spiegel saw Mason for several sessions while Wilbur was on vacation.