  The limits of frequency resolution in nano-NMR experiments have been
discussed extensively in recent years. It is believed that there is a crucial
difference between the ability to resolve a few frequencies and the precision
of estimating a single one. Whereas the efficiency of single frequency
estimation gradually increases with the square root of the number of
measurements, the ability to resolve two frequencies is limited by the specific
timescale of the signal and cannot be compensated for by extra measurements.
Here we show theoretically and demonstrate experimentally that the relationship
between these quantities is more subtle and both are only limited by the
Cram\'er-Rao bound of a single frequency estimation.
