Thank you for your advices and interest to my problem!

Let me tell the whole story. :) One of our sites exists already 5 years but two years ago some changes occured in my company and we had to register new domain. Two domains are aliases to one site, so they have the same IP. We pay attention mostly to the new one but I still don`t want to loose the old one because it has good SE rankings and a lot of backlinks.

Sorry for doubts. Sometimes advices are too theoretical and don`t deal with real life. I suppose you have great experince so please answer on these questions: Did you know cases when both sites were penalized in such situation? Will Google consolidate PR and backlinks (If I use 301)?

Thank you!