So because of the yin-yang one will be offensive and the other defensive I'm guessing.
I'm not sure why people are complaining that reshiram being the bulky one is such a bad thing, he would only be weak to rock, ground and dragon, which is not that bad
Stealth Rock weakness is big, along with being weak to Earthquake and Stone Edge. It does resist Bug, Electric, Fire (4x), Grass (4x), and Steel. However, many good attacking types hit it for neutral: Dark, Fighting, Ghost, Ice, Water, amongst other less common types.
Dragon/Fire is such a good offensive typing, so that's why we all think it would be a waste of potential. I'm thinking (hoping) one will be physically based (Zekrom) and the other special (Reshiram). Give Zekrom Dragon Dance, Outrage, Wild Volt, and Earthquake. Reshiram can run Draco Meteor / Fire Blast / Earth Power / filler, or maybe a special version of Dragon Dance.