Combining simultaneous measurements of the same signal for better SNR?
Coherent Gain happens when you have copies of the same signal but different noise. If the noise is independent in each copy, the gain should be $10log N$ dB where $N$ is the number of copies. For a well crafted measurement system, additive electronic noise should be independent. Things like ambient noise will have some correlation provided a reasonable distance between sensors and $5logN$ is a reasonable lower expected gain.

If you don’t see at least $5 log N$ dB, then there is probably something wrong. Coherent Gain works. If it didn’t, many here would not have jobs.  

You might try cross correlating pairs of your copies as a diagnostic. Without knowing your data, I cant say with certainty that you should see a well defined peak but the absence of a well defined peak would be a cause for concern