Timid is a better nature for Reshiram; Hasty is usually for mixed sets, and Reshiram's moves are all special.
Zekrom - Choice Band
Reshiram - Choice Specs
Choice Scarf is also viable for either of them, but that forfeits the power boost for a speed boost, so I would choose Scarf > Band/ Specs only if your team does not have a faster Pokemon than Reshiram/ Zekrom.
For Reshiram, I would recommend this Choice Specs set:
Reshiram @ Choice Specs
Ability: Turboblaze
EVs: 252 Sp. Atk, 252 Speed, 4 HP
Nature: Timid
- Earth Power
- Blue Flare/ Fusion Flare
- Dragon Pulse
- Draco Meteor
I don't recommend Focus Blast due to its low level of accuracy, and the only thing Reshiram needs coverage for is Heatran, whereby it can be dealt with by Earth Power. Often Heatrans carry Air Balloons, which can be annoying, so if you want, you can go for a this set:
Reshiram @ Life Orb
Ability: Turboblaze
Nature: Timid
- Earth Power
- Blue Flare
- Dragon Pulse
- Roost
This frees up your moveset, and Roost replenishes Life Orb recoil.
As for Zekrom:
Zekrom @ Choice Band
Ability: Teravolt
EVs: 252 Atk, 252 Speed
Nature: Adamant/ Jolly
- Outrage
- Dragon Claw
- Bolt Strike/ Fusion Bolt
- Crunch
Stone Edge has a low accuracy doesn't cover anything; Zekrom is better off facing an Ice type with its STAB moves, and the only thing Zekrom cannot cover with its STAB moves are Ferrothorn and Excadrill, both of which have a Rock resistance.