The ornamental design for a lockingdevice for double French door, as shown and described.
FIG. 1 is a perspective view of my locking device for double French door;
FIG. 2 is a bottom view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a rear view thereof; and,
FIG. 7 is a front view thereof.
The broken lines showing of locking device for double French door is included for the purpose of illustrating portions of the article and forms no part of the claimed design.