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)