Most unis offer them. Sometimes they're related and sometimes they're not (like B Law/B Art). You can visit any unis website and they'll have all of their courses listed, including the double degrees they offer.
I'm not an expert but I believe you just do each course's units simultaneously.
So if Person A is doing a B Arts they will do 4 art units in a semester, but Person B doing B Art/B Law will do 2 art and 2 law units per sem - but I feel like you can also customise this? Depends on the uni I guess.
Feel free to correct me if anyone thinks this info is incorrect