  We analyze the relation between the emission radii of twin kilohertz
quasi-periodic oscillations (kHz QPOs) and the co-rotation radii of the 12
neutron star low mass X-ray binaries (NS-LMXBs) which are simultaneously
detected with the twin kHz QPOs and NS spins. We find that the average
co-rotation radius of these sources is r_co about 32 km, and all the emission
positions of twin kHz QPOs lie inside the corotation radii, indicating that the
twin kHz QPOs are formed in the spin-up process. It is noticed that the upper
frequency of twin kHz QPOs is higher than NS spin frequency by > 10%, which may
account for a critical velocity difference between the Keplerian motion of
accretion matter and NS spin that is corresponding to the production of twin
kHz QPOs. In addition, we also find that about 83% of twin kHz QPOs cluster
around the radius range of 15-20 km, which may be affected by the hard surface
or the local strong magnetic field of NS. As a special case, SAX J1808.4-3658
shows the larger emission radii of twin kHz QPOs of r about 21-24 km, which may
be due to its low accretion rate or small measured NS mass (< 1.4 solar mass).
