The ornamental design for a double extension for a tree stand, as shown and described.
FIG. 1 is a right side elevation view of a double extension for a tree stand showing my new design, the opposite side being a mirror image of the shown;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof; and,
FIG. 6 is an isometric view thereof.
The broken lines in FIG. 6 of the drawings depict a portion of a tree stand as environmental subject matter only and form no part of the claimed design.