nostr:npub1q8nhcxa8t9zfk06ggqwpwcr64znkxvnwt2wgzqdtga920g355p4qdfkct8 Posts are signed with a single private key, so generally no. Relays may implement any custom functionality, of course, but it wouldn't affect the rest of the network. Someone with the key can still post to any other relay, effectively making 2FA useless.