FranticFish - 10:51 am on Dec 29, 2011 (gmt 0)
It's better to start a new thread!
I'm talking about when the domain is still with the registrar and you just forward the URL to a page on your other site. So if someone goes to dogs.tld they land at petproducts.tld/dogs but it looks like they are at dogs.tld.
If this is a standard forwarding feature offered by a big web host then it sounds like a 100% height, 100% width frameset to me. This is a very common method and as far as I know does no harm whatsoever, and no link benefit is passed from the framing domain to the framed domain. You can even build some links to the framing domain and get it indexed. The only drawback is that users never see the true url they're on so they can't bookmark it / forward to a friend etc.
View source on both domains. If you see different code, then this is what is happening.
But, if you see the same code at both urls then the new domain is aliased to the old one - in other words, both resolve to the same webspace. Google sees two sites with the same content and the same owner. This IS a potentially huge problem.