The ornamental design for a dual planthanger, as shown and described.
FIG. 1 is an environmental perspective view of a dual plant hanger showing my new design;
FIG. 2 is a front elevational view with the rear view being a mirror image thereof;
FIG. 3 is a right side elevational view with the left side being a mirror image thereof;
FIG. 4 is a top view thereof; and,
FIG. 5 is a bottom view thereof.
The broken line showing of the lower hooks, support line, plants, and baskets in FIG. 1 is for illustrative purposes only and forms no part of the claimed design. The broken line showings in FIGS. 1-3 merely discloses the types of attachments that may be used with the hanger and forms no part of the claimed design.