Calculating attenuation of coax cable run
For an 1-input-to-2-output splitter, you can consider a 3dB attenuation (50 % of the input signal to each output) excluding losses and reflections on connectors.For a coupler, it depends on rated de-coupling ratios (translate percentages to dB yourself).

In any case, only trying your solution in vivo will give you a final (and correct) answer.

As answered to your previous question about solutions in common, two media converters and two fibers between them is the best solution for you. Do not use long copper cables (conductive runs) if you are not heavily experienced in EMI protection/susceptibility.