dBm addition question - powers, logs
As the difference in levels between the two signals is 'large' (more than 20dB, more than 100x in power), you can pretty much ignore the smaller signal. 

The total power of two signals of -25dBm and -47dBm will be 'about' -25dBm.

If you want to convert both to power, add, and convert back to dB to get the exact figure, as Transistor has done, then you get -24.975dBm. The difference of 0.025dB is insignificant. 

You'd have to make very careful measurements in a wired system to believe measurements to that precision. In an antenna system, you can be many dBs adrift without even trying due to direction of arrival, impedance mismatch, path loss, proximity effects etc.