We went over this question pretty thoroughly in a thread called "commercial checkride" in the Flight Training section.
To cut to the chase you can't log it. There was a part 91 accident recently in which an MEI was in the right seat of a light twin supposedly giving "instruction" to the pilot/owner. In fact he was time building. The FAA pulled his certificates and violated him (and the pilot/owner). The FAA ruled that to log PIC time he had to be giving instuction. If the owner had come to him and asked for dual to improve his skills in some way or to meet some currency requirement, fine. But the MEI solicited the owner to fly right seat and "we'll just call it dual". That's a no-no.
Second, if the flight was a part 135 flight you would need to meet the requirements of part 135 to act as PIC on the flight. You would also have to be authorized under part 135 to give instruction. Frankly you are nothing more than a passenger. I have unlogged right seat time in a KingAir 100. It was great fun, but from a loggable time standpoint, worthless.
Last edited by WEACLRS; 04-03-2006 at 12:17 PM.