It is very common in Korea and Japan to charge extra for 2nd guest (or the other way around; if 1 guest only you get a discount) which is actually fair.

For Hilton Honors members at Hilton properties this is never a problem due to the "spouse stay free" policy; you just book a room for 1 guest and should have no problems when 2 guests check-in. I always put a note in the reservation "2 guests - HH spouse stay free" - I have never encountered any problems.