We had that for awhile. Web site started out as:
Later, we also began using:
They were both active domains with the same mirrored content - though we hardcoded all the links to the "companyname.com" site for the shopping cart.
Google figured out that they were the same and counted all incoming links as being shared (we had a mix of links to both domains), and it only showed our older domain.
Every now and then we would see some of our newer domain pages showing up in Google - usually because we had new content that freshbot picked up.
Because of a concern that google might mess up at some point in the future and think we had duplicate content, we put a 301 on our old domain.
Google was very quick to completely drop us from the results, but it took about three months to get back into the results with our new domain (despite having all the links that were already in place!). And, once we came back, we were treated as being "new" content (even though we had PR5).
All of this happened at the time of the Florida/Austin updates which would have also hit us rather hard. Hopefully this update hurries up because it should put us back where we belong (even though I'm biased, I think we do reasonably belong back near the top for a few keywords)...
So, to answer your question - google did just fine with the way we previously did it. However, google will only list one domain name - but you'll likely get credit for all incoming links. Down the road, it'll be a major hassle to switch away from that method (unless you simply 301 all the domains that google isn't listing and stick with the one that google lists).
Hope that helps.