Mount two spools of filament on your aluminium plate (6mm) Prusa i3.
Print two of the dual plate mounts and 4 of the spindle holders.
Cut four lengths of M6 and two lengths of M8 threaded rod ~ 150mm long.
Fit two of the M6 threaded rods to each plate mount with M6 nuts and washers. Fit a spindle holder to the other end of each M6 threaded rod.
Print four spool adapters ( http://www.thingiverse.com/thing:60723 ), fit two to each spool along with the M8 threaded rod and drop that into the spindle holders.