  We report the realization of a matter-wave interferometer based on Raman
transitions which simultaneously interrogates two different atomic species
($^{87}$Rb and $^{85}$Rb). The simultaneous aspect of our experiment presents
encouraging preliminary results for future dual-species atom interferometry
projects and seems very promising by taking advantage of a differential
acceleration measurement. Indeed the resolution of our differential
accelerometer remains lower than $3.9\times 10^{-8}g$ even with vibration
levels up to $1\times10^{-3}g$ thanks to common-mode vibration noise rejection
. An atom based test of the Weak Equivalence Principle has also been carried
out leading to a differential free fall measurement between both isotopes of
$\Delta g/g = (1.2 \pm 3.2)\times10^{-7}$.
