I want to handle two rabbits in the backyard.
What factors are important for rabbits safety and health?
(I'm only looking for answers relating to safety and health. I have asked about happiness and life quality separately here)
Some important factors when creating a safe and healthy outdoor living place for a rabbit include:
Protection from predators. This includes protection from being forced to stay in close proximity to a predator without a means of hiding. Even if the rabbit can not be physically harmed, the stress of being unable to escape can be harmful. Provide a nest box or house for your rabbit can hide in.
Protection from the elements. Rabbits obviously need protection from the cold, but heat is actually even more dangerous and likely to cause harm. When the temperature is over about 80f (a little less than 27° Celsius), you will likely need to take special efforts to keep your rabbits cool, like installing fans, providing ice several times a day, etc.
They need to be able to stay dry when it rains, to avoid standing too long on very wet or muddy ground, and access to shade at all times. They also need adequate airflow AND protection from strong winds or drafts.
Ability to express normal behaviors Rabbits need to chew, need room to run and jump, and the enclosure must be tall enough for them to sit up on their haunches with ears up without hitting the ceiling. They need a protected area to rest. They need an area to relieve themselves that is not too close to their food and water.
Avoid physical pain/illness/injury Rabbits need a comfortable place to sit, and should not be left to stand or sit on a wire-bottomed cage all day. This can result in 'sore hocks' (foot injury that is prone to life-threatening infection). They also need a clean environment. A buildup of urine or stool in the hutch can result in your rabbit getting sick or developing an infection of the skin or respiratory tract. Design the hutch so that it's not too hard to clean up.
Appropriate nutrition Rabbits need access to fresh water and unlimited fresh hay. They also need fresh greens. Be aware that you need to be very careful if you store your hay outdoors, it can easily grow toxic mold if it gets damp.
Appropriate socialization A rabbit that is kept outdoors gets a lot less of its social needs met by humans, so it's great that you are planning to get two rabbits. You should get rabbits that are bonded, or socialize them so they can be housed together if at all possible. If this is not successful and they don't get along well enough to share an enclosure you should set up their housing so they can interact but not fight, perhaps separated by a wire mesh. Rabbits don't usually fight to the death or anything, but can potentially hurt each other, or just suffer a lot of stress over territory disputes.
Physical Security Depending on where you live, or the ages of children in your neighborhood, you may want to place a padlock or combination lock on your rabbit's hutch to prevent interference, theft, even a kid opening the hutch and accidentally letting your rabbit escape.