Actually you have to convert dB to linear using: linear = 10^(dB/10)
So, 18.4dB = 69.18 linear
and 11.8dB = 15.13 linear
we get (69.18*15.13) / (69.18+15.13) = 12.41 linear
in dB = 10*LOG(12.41) = 10.9 dB
There are a lot of discussions on the Internet about this topic (also in the link above), and myself I gave up finding the right answer.
Is somehow related to the difference between S/No and C/No.
Very often the published equations looks like: