  With quantum calculations, we have investigated the multiphoton nonsequential
double ionization of helium atoms in intense laser fields at ultraviolet
wavelengths. Very surprisingly, we find a so-far unobserved double-circle
structure in the correlated electron momentum spectra. The double-circle
structure essentially reveals multiphoton Rabi oscillations of two electrons,
which are strongly supported by the oscillating population of a certain doubly
excited state and by the oscillating double ionization signals. This
two-electron multiphoton Rabi effect provides profound understandings of
electronic correlations and complicated multiphoton phenomena and is expected
to be a new tool for broad applications, such as quantum coherent control.
