You can use:
RewriteCond %{THE_REQUEST} /index\.php\?page=([^\s&]+)--([^\s&]+) [NC]
RewriteRule ^ /%1/%2? [L,R]
And if you need to go in another direction:
RewriteCond %{SCRIPT_FILENAME} !-d
RewriteCond %{SCRIPT_FILENAME} !-f
RewriteRule ^([^/]+)/([^/]+)/?$ index.php?page=$1--$2 [L]
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