Each owner of the duplex is responsible for the cost of replacing their roof up to the mid point of the dividing wall.
BUT the OC can refuse permission to change the style of roof because a change requires 66% in favor of said change. If the owner just replaces the roof with like I don’t consider that requires any vote as it’s just maintaining the property.
The OC can control the look of the property and veto any attempt to change the appearance without 66% voting in favor of the change using bylaw 11.
The relevant bylaws are those that were registered in 1988 not the new ones.
If both owners wanted to replace the roof with colorbond they would still require a vote of 66% in favor of the change because like you said they are changing the appearance of the complex.