  At BNM-SYRTE laboratories in Paris two 133Cs fountains and a 87Rb one have
been already demonstrated. This paper illustrates the construction of a 87Rb
and Cs dual fountain realized within the same structure to work simultaneously.
Up to now only the Cs part is under operation but Rb part, already completed,
will be soon operational. The experiment has been projected to improve present
fountain frequency standards and to perform a more precise laboratory test (at
10E-16 level) on a possible time variation of the fine structure constant
alpha, as recent cosmological evidences strongly motivates.
