Raid difficulty in solo vs duo/group

Hello,

My buddy and I are getting close to 40 and from the sounds of things, Yael, Vox and Nag are all soloable at 40, especially on d0. I am curious as to whether they are more difficult if you show up as a duo? Like more HP, abilities, adds etc, or if those are all static and just linked to difficulty setting?

For ref, I am War/Mnk/Sha and he's running Zerker/brd/rng so I expect to have no trouble tanking and imagine his damage + a charmed pet should make this easy?