Under old CAA Rules and now I am led to believe under JAR Rules you cannot both log PIC in a single crew Aircraft, and as I am sure your not going to be hour building in anything bigger than a light twin then you have your answer. However if you are using FAA Rules then double logging as mentioned, one under the hood one acting as safety pilot etc is allowed, But as I understood it when the rule as was pointed out to me by FCL before I went to the states to hour build, You could/cannot dual log PIC if you use those hours towards a UK/JAR Licence.
If I am wrong don't shoot me but as I said that is how I see the rules.