Ouch. You're talking about a rewrite-- or technically a proxy, since they're different domains. One of the few things robots and humans have in common is that they don't know they've been rewritten and have no control over the process. So the search engines will look around and say "Gosh, this 'dogs.tld' domain sure looks an awful lot like the 'cats.tld' I've been indexing for years" and whip out the Duplicate Content stamp.
If you can't do anything with the new domain yet, it seems safer to leave it back-burnered. Just remember to keep the registration paid up. If you absolutely must put the new name "out there", at least make it a redirect, where visitors can see that they're being sent somewhere else. The search engines will shrug and move on.