Well let's get technical and shit then.
Pereira, F. C., et. al. (2006). Methamphetamine, morphine, and their combination: acute changes in striatal dopaminergic transmission evaluated by microdialysis in awake rats, Annals of New York Academy of Science, 1074:160-73
Uemura, K., Sorimachi, Y., Yashiki, M., Yoshida, K. (2003). Two fatal cases involving concurrent use of methamphetamine and morphine, Journal of Forensic Science, 48 (5)
Ginawi O. T., al-Shabanah O. A., Bakheet S. A. (1997) Increased toxicity of methamphetamine in morphine-dependent mice. Gen Pharmacol, 28 (5):727–31.
Funahashi, M. Kohda, H. Hori, O. (1990). Potentiating effect of morphine upon d-amphetamine induced hyperthermia in mice, Pharmacology Biochemistry & Behaviour, 36: 345-50
Landa, L., Šlais, K., - Šulcova, A. (2004). Cross-sensitization with morphine to methamphetamine effects on locomotion in mice. Psychiatrie, 8 (1) :53
Some of these are free online, if you can't find an article and want to read it, ask me and I'll use my university account to transcibe it here.
Basically it seems the main danger is hyperthermia, and this can cause death at levels far below what would normally be considered lethal if either morphine or meth were consumed seperately. This enhanced toxicity is mediated through opioid receptors, 1b-adrenergic receptors, and dopamine-serotonin receptors. Other risks include tachycardia and hypertension.