If the relation between T and M runs exclusively via E, then in both MZ and DZ twins, the cross-trait cross-twin correlation between M2 (the moderator of twin 2) and T1 (the trait of twin 1) is 0, just as the correlation between M1 and T2 is 0, i.e., rm2,t1 = rm1,t2 = 0. If the correlation between T and M runs via C, then the correlation between M2 and T1, and between M1 and T2, is in both MZ and DZ twins calculated as \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \sqrt {.3*.2} = .24 $$\end{document}. Finally, if the correlation between T and M runs via A, then this correlation is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \sqrt {.4*.15} = .24 $$\end{document} in MZ twins and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \sqrt {.4} *.5*\sqrt {.15} = .12 $$\end{document} in DZ twins.
