Get Two Sizes of Header Image

My theme uses header_image(); function to echo out a header image img src="..." it is set to 200x50px with add_theme_support(...); - all good here. My problem comes when I want to get the url of retina sized version of the image. Tried get_header_image(); but it doesn't accept any parameters so can't pass custom size (400x100px). How can I (using WP's header_image();) get the url of the retina-size version (400x100px) of the standard (200x50px) header image?

If it's not possible maybe we could try setting the default image demensions to 400x100px and resizing it down to 200x50px, and getting the url of a resized image?

Basically, I want to get two urls with the use of wp's core function - header_image();, one with 200x50px dimensions and one with 400x100px dimensions.