Using a .com and another .com domain and mirroring the sites exactly has been covered many times before. It's generally a no no. Slight changes between the sites seem acceptable and not likely to get you into trouble with the SEs.
With filtering in uk directories/SEs, there are lots of good reasons to use a .com and a .co.uk
Who here is using this strategy and what have you discovered? What are the benefits? What are the downsides?
18 months to 2 years ago I built a .com site for a UK company and when I realised that it wouldn't get into most UK se/dir made an almost identical co.uk site. The .com site has been promoted heavily and updated quite often, the co.uk site hasn't been touched, in fact it hasn't even been checked.
I promoted the .co.uk site only into the UK areas that wouldn't take .com - including Excite uk
There have never been any problems and it still continues to get traffic.