Edit: Looking at the IPv6 view, I see no reason why OnionCat and GarlicCat couldn't peer. It's just that gateways would be needed. And that could chew up humongous bandwidth.
% dig whois.dn42 @resolver.nic.dn42 any +short
% openssl x509 -in /etc/ssl/certs/dn42_Root_Authority_CA.pem -noout -text
X509v3 Name Constraints: