Understanding Cutkosky's Rules for Bhabha Scattering and the Optical Theorem

[ndung200790]

Please teach me this:
When we use cutting rule for Bhabha scattering to cut the Feymann diagram to parts.Why we know the two parts are complex conjugation of each other,so we have optical theorem:
2ImM(s)=... M.M*...
Thank you very much in advanced

 

[ndung200790]

How to calculate Feymann diagram with two loops which have common propagator(a common segment of diagram).And how to use cutting rule(Cutkosky's rules) those two loops(It seem to me that in this case there are 3 propagators being cut in the two loops because there is common one)?