I can only suggest workarounds
You could try creating a common user / password combination on both of the network shares, and use that user for the win_copy task. Its not something I have tried, but it might work.
Otherwise you could split up the transfer and use smbclient to transfer files to your ansible controller and then win_copy them from the controller to the destination share. If the files are large I'd consider using a web server and downloading using `win_get_url` as for large files this is much faster than win_copy.
Hope this helps,
Jon