The ornamental design for a portable dual tubegrainauger, as shown and described.
FIG. 1 is a perspective view of a portable dual tube grain auger according to the present design;
FIG. 2 is an elevational side view thereof;
FIG. 3 is the other side elevational view thereof;
FIG. 4 is an end view thereof;
FIG. 5 is the other end view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a perspective view of a second embodiment of a portable dual tube grain auger according to the present design;
FIG. 9 is an elevational side view thereof;
FIG. 10 is the other side elevational view thereof;
FIG. 11 is an end view thereof;
FIG. 12 is the other end view thereof;
FIG. 13 is a top view thereof; and,
FIG. 14 is a bottom view thereof.
The broken lines in the figures are included for the purpose of illustrating portions of the portable dual tube grain auger and form no part of the claimed design.