Rms value of sum of two sine waves?

question is not clear. RMS is .707 of peak. If you're asking about sums of currents or voltages represented by sine waves and they don't have the same frequency then the sum's RMS value is the square root of the sum of the squared amplitudes of the two waves. The squared amplitudes are proportional to the power in each current or voltage and the result represents the sum of the powers.

If the two sine waves have the same frequency then the sum's RMS value is the sum of the two RMS values. The physical circuit has to add power to the signal. (if the amplitudes are equal the resultant will have 4 times the power)