Currently the specification doesn't mention anything related to DPI support.
It seems that people are wrongly using the 128px icons for the 64@2 icons but they are not the same icons.
Here are more explicit informations about this problem:
http://blog.elementary.io/post/124193007916/whats-up-with-hidpi-icons
To solve this problem i propose to allow a dpi property that way:

<icon type="remote" width="64" height="64">http://example.com/icons/foobar.png</icon>
<icon type="remote" width="64" height="64" dpi="2">http://example.com/icons/foobar@2.png</icon>

The default dpi value should be 1 so it's not mandatory to set it.