Is it more fatal to have two morphological types of leukemia (m4&m3)?

Each case is unique. This is obviously a serious condition, and your oncologist will examine all available treatment options and assess the patient's risk factors and response to therapy. Discuss this with him or her.
Wow... I have never heard of simultaneous m3 (acute promyelocytic leukemia) and m4 (acute myelomonocytic leukemia)! some m3's may have monocytic morphology (variant m3). The only other possible scenario I can think of is m3 treated with chemotherapy and then followed after years by therapy-related m4, but then they wouldn't be concomitant. Theoretically, 2 cancers are worse than 1, but I doubt it...