There is some internal curl error (CURLE_FAILED_INIT) when you are trying to use just copied curl handle in curl_multi_add_handle(). I have checked the same problematic PHP code but with little difference: instead of creating curl’s copy I have used the original one (template). As I expect code works without any error. I think curl_multi_* along with curl_copy_handle() is still raw and needs some improvements.
With best wishes
Vitali Simsive