  We aim to obtain new insights into the internal dynamics of the cluster Abell
1240, showing the presence of two roughly symmetric radio relics, separated by
~2 h_70^-1 Mpc. Our analysis is mainly based on redshift data for 145 galaxies
and on new photometric data. We also use X-ray data from the Chandra archive
and photometric data from the SDSS (DR7). We combine galaxy velocities and
positions to select 89 cluster galaxies and analyze the internal dynamics of
the Abell 1237 + Abell 1240 cluster complex. We estimate similar redshifts for
Abell 1237 and Abell 1240, <z>=0.1935 and <z>=0.1948, respectively. For Abell
1237 we estimate velocity dispersion sigma_v~740 km/s and a mass M~6 10^14
h_70^-1 M_sun. For Abell 1240 we estimate a LOS sigma_v~870 km/s and a mass
range M~0.9-1.9 10^15 h_70^-1 M_sun, which takes into account its complex
dynamics. Abell 1240 is shown to have a bimodal structure with two galaxy
clumps roughly defining the N-S direction, the same one defined by the
elongation of its X-ray surface brightness and by the axis of symmetry of the
relics. The two--body model agrees with the hypothesis that we are looking at a
cluster merger occurred largely in the plane of the sky, with the two galaxy
clumps separated by a rest-frame velocity difference V_rf~2000 km/s at a time
of 0.3 Gyrs after the crossing core, while Abell 1237 is still infalling onto
Abell 1240. Chandra archive data confirm the complex structure of Abell 1240
and allow us to estimate a global X-ray temperature T_X=6.0+-0.5 keV. In
agreement with the findings from radio data, our results for Abell 1240
strongly support the "outgoing merger shocks" model to explain the presence of
the relics.
