Moved Post: Clarification on Rules - Any one
 

We are 3 people riding 2 bikes on a long trip from Nepal to London

Situation
A Bike owner (Mr. X) on this long trip which the two of us plan to do together would like to join us in another country half way after the trip is thro due to time constraints.
In the mean time can my pillion rider RIDE the bike with Carnet papers and bike registration papers belonging to the owner of the bike (Mr. X) who will join us later half way thro various countries.

Kayjay :scooter:

I am not sure there is a definite answer to this, it might depend on the attitude of indivdual customs and police officers you encounter along they way. It would help if you can minimise the number of border crossings done by the pillion without the owner being present.
You don't mention your route but if there is any shipping or flying of the bike involved it would be best if the owner was present as these clearances are always a lot more difficult than land border crossings.
A covering letter from the owner giving permission to use the bike, perhaps with a photocopy of his passport photo page on it might help.

crossing borders in a vehicle not registered to yourself is not an exception: most truckdrivers don't own their own truck... However, the owner's cover letter (preferably notarized or otherwise royally endowed with important looking stamps) will go a long way.

It is also part of the information you supply while applying for the carnet, so most will be covered there.