Swapping seats in flight might upset the cabin crew (and possibly others), but swapping boarding passes for the second leg should be quite feasible. The cabin is cleaned at Singapore, or wherever, and possibly more important, there is a new cabin crew so there is no risk of being remembered from the first first leg. Avoid the fast track boarding; go through the non-privileged checks in a group each with a boarding pass that matches their passport, and then swap as apropriate between there and the aircraft door. The only problem on board might be remembering to respond to your friends name rather than your own.