To Mosaic two Images using Quick Mode and interactive Feathering

Select the Define Seam tool.
Select the first raster.
Select the second raster.
From the Tools Settings box, set:
Seam Type: Quick

Feathering: By Input Point
Interactively define the feathering width.
A blue outline surrounds the image.
Enter a data point to begin the mosaicking process.

Note: The images now form a seamless mosaic!