A passenger caught out with having purchased extra tickets under false pretenses would void the requirement that the extra berths be held and not resold. Whether the conductor would actually resell a bogusly-procured berth is another issue. And of course, he'd have to find out first.

So OP, if you try this and get control of all berths in a compartment, it's probably wisest to just show your two real tickets and ID's when they come to the compartment and ask for, and play dumb about the other two berths, especially if the family names of the other two don't match your own. Train staff might just assume they are getting on further down the line, per JPDM's example.