The ornamental design for a double-fiddle stand, as shown and described.
FIG. 1 is a front perspective view of a double-fiddle stand showing the new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a front perspective view of the design in the context in which it is intended to be used, the broken lines forming no part of the claimed design; and,
FIG. 9 is a perspective view of the design in a collapsed configuration and in the context in which it is intended to be used, the broken lines forming no part of the claimed design.