This question is difficult to answer without more information about the process conditions that need to be duplicated in the scale-down.
Starting with the basic conditions as described and looking at different measures of mixer design and mixing intensity, several options exist.  In each case, the impeller diameter to tank diameter ratio (a 6.3 inch impeller in a 39 inch tank and a 2 inch impeller in a 12.5 inch tank) is 0.16, which is small, except for the high speeds for each mixer.  The impeller tip speeds cannot be matched in both sizes, with either one or two impellers: 1,155 ft/min at 700 rpm (minimum speed) with a 6.3-inch diameter impeller and 830 ft/min at 1,585 rpm (maximum speed) with a 2-inch diameter impeller.
Other measures of mixing intensity, such as torque per volume and power per volume, also fail to overlap in the two sizes, with less intensity in the small vessel with one impeller than in the large vessel with two impellers.
About the only combination that seems to provide some equivalence would be with a 2.5-inch diameter impeller in the small scale.  With that impeller operating at the maximum speed (1,585 rpm) and the two 6.3-inch diameter impellers operating at the minimum speed (700 rpm) tip speed and other measures of mixing intensity become more nearly the same.
The basic answer to the question is to use a single 2.5-inch diameter impeller in the small-scale tank looks like the best option.