Well, at least in my understanding, any use of EC based crypto necessarily needs to choose/agree a elliptic curve. So I *assume* `set_tmp_ecdh_curve` would set that curve for both. I haven't looked into the OpenSSL sources for this.

Regarding curve selection with the TLS protocol in general:
http://crypto.stackexchange.com/questions/11311/with-tls-and-ecdhe-how-does-curve-selection-work

This is what others do:
http://dovecot.org/list/dovecot/2013-July/091318.html

And this contains some info bits about "single use":
http://linux.die.net/man/3/ssl_ctx_set_tmp_dh