Not working with Retina @2x

To have alternative retina images is now more and more common.

The retina image normaly has the same file name, but ads @2x.
Ex: filename.jpg and filename@2x.jpg

The problem is that since the retina image is not in the media library, it can not be cropped.

If I crop an image with size x I would like that the exact same cropping also occurs for the retina version on that size.

How can I solve this?

One common plugin for Retina is:
http://wordpress.org/plugins/wp-retina-2x/

But many others work the same and just ads @2x in the filename.

https://wordpress.org/plugins/post-thumbnail-editor/