I would be hesitant to have 2 together. 2 bucks will usually fight. Intact does can fight as well and they tend to be more territorial. Being siblings or raised together from a young age does not matter once hormones kick in. Getting them spayed or neutered really helps with bonding, but then you can't show them. I would get one now and then consider a second one later on, but keep them in separate cages.
As for what to ask breeders, I can't really help with what to look for in a show rabbit. You do want a healthy rabbit and one both you and your daughter can handle. Look the rabbit over checking the teeth, eyes, ears, nose, nails (don't want them too long plus nail colour can be a DQ). Any sort of discharge can be a sign of illness. Also check for signs of mites and other skin issues (it may look like dandruff or crustiness). Since your daughter is the one who will be showing the rabbit, she needs to be able to handle it easily, so get her to handle the rabbit once you have looked it over.
Definitely be prepared to bring a rabbit home. Have a cage set up at home and supplies ready. It doesn't have to be totally set up, but should be easy enough to get ready if you do take one home. Bring a suitable sized carrier with bedding and food and water dishes, some hay inside is good as well. The breeder you get the rabbit from should have some pellets for you. Make sure to get the pedigree there as it can be hard to get it later from some breeders, just better to get it done while you are there.