hello everyone.
i agree with asif. When you are adding power using a power coupler, it is log addition. Simply convert power at inputs into miliwatts and then add. Suppose you take two lasers of 10 dbm power which means 10 milliwatt each. It should give you 40 milliwatts at the output which is equal to 13dbm.
I believe asif has shown you correctly.
regards