Multiple pets
If you have multiple pets, most often you'll find, they will live together in harmony. However, there are certain problems which can, and do, arise. In the wild, dogs will live in packs, if you have more than three dogs, effectively, you have a pack.
It can be difficult training just one dog, when dogs are living together; it is even more difficult to train them. Simply because they do not create bonds with their owners, a single dog normally would. Therefore, it is important to spend time regularly with each dog. Plus, when you do spend that one on one time with them, training needs to be involved.
Trying to train multiple pets, together, will not be productive, it is important to train each dog individually, if you really want them to take notice of your commands. Multiple pets can also mean problems with fighting; one Alpha male and one Alpha female in the house will not produce problems.
The animal world is very simple that way; any two animals of the same sex will naturally work out their 'pecking order'. However, ranks do shift, and can change, so it's important to keep an eye on things. Especially if one of the animals has been out of the 'pack' for a while, they will normally need to re-establish themselves.
Normally, multiple pets will successfully establish their own ranks, without too much harm done. Other times, you may introduce a new animal and they just can't seem to get along.
If the fighting becomes particularly nasty, all the animals will be at risk. This is not an acceptable situation, miserable for the animals and the owners concerned. In a situation like this, is often best to find your new addition a new home.
Safer for you, them, and your other animals, some people will not do this and choose to separate the animals. If it's dogs, you are actually putting their lives at risk. Accidents do happen; it's hard to keep animals apart without strict regulation. If you fail, and they do get together, dogs will fight to the death, remember that.