  Motivated by experimental advances [e.g. A. Ga{\"e}tan {\it et.al.} Nat.
Phys. 5 115 (2009)] that the collective excitation of two Rydberg atoms was
observed, we provide an elaborate theoretical study for the dynamical behavior
of two-atom Rabi oscillations. In the large-intermediate-detuning case, the
two-photon Rabi oscillation is found to be significantly affected by the
strength of the interatomic van der Waals interaction. With a careful
comparison of the exact numbers and values of the oscillation frequency, we
propose a new way to determine the strength of excitation blockade, well
agreeing with the previous universal criterion for full, partial and none
blockade regions. In the small-intermediate-detuning case we find a
blockade-like effect, but the collective enhancement factor is smaller than
$\sqrt{2}$ due to the quantum interference of double optical transitions
involving the intermediate state. Moreover, a fast two-photon Rabi oscillation
in $ns$ timescale is manifested by employing intense lasers with an intensity
of $\sim$MW/cm$^2$, offering a possibility of ultrafast control of quantum
dynamics with Rydberg atoms.
