Sorry WoolyMarmot, but this is topic that seems to get people relatively heated up. It's a touchy subject, especially when we (including BotanicalStig, I'm sure) are just looking out for these animals and have devoted a lot of time, care, and research into them, only to have seen and heard of a number of incidences where individuals were too stubborn or misguided to realize the consequences until they were too late. To us, it's just very hard to understand that someone would risk another creature's general well-being and potentially their entire life, so the subject usually becomes a hot matter. We are just concerned about the animals, and it's difficult for us to know that we are unable to do anything other than write what we can to hopefully convince others otherwise.
And I'm sorry to hear about your current situation. I know that you must really want one, and that it must seem unfair if your brother gets one and doesn't let you have a part in its care-taking. But I think this is one of those situations where you have to make a mature and responsible decision: would you rather have two bearded dragons that may be miserable and constantly in harm's way just so that you could have one for yourself, or would you rather give one bearded dragon a chance at being completely safe and happy even if you couldn't be a part of it life? And who knows? Your brother may be willing to share, or he may get bored of taking care of the bearded dragon after the first few days of interest (not saying he's like this, but some kids are really just interested for a bit, until they realize it takes hard work to care for them) and maybe you could take over later. I know it's hard to pass up the opportunity, but wouldn't you much rather get a bearded dragon when you are sure you can provide for it as best as possible, meaning it's separated and with its own, wide open enclosure? It might have to be a sacrifice you'll have to consider.
As for the whole issue about your parents not listening to you, that is really unfortunate. You sound like you know what you are doing and you definitely have shown you are willing to research and try to do what's best. If it helps to convince them to listen to you rather than the pet store owner, ask them to consider doing some research here first (and I'm glad you will show them the thread I have provided), as most, if not all, of us on this site take care of or have taken care of a bearded dragon and/or other reptile in our lives, and do so for the sake of the animal and for companionship. I'm not saying that all pet store owners or employees are like this, but have your parents consider whether they really want to be listening to a pet store owner or employee who is caring for the animal for the sake of selling it; some pet store owners or employees will even tell misinformed customers that it is okay to house two or more of them together (even when they know it is not okay), because they want you to buy more of them. We have nothing really to gain from telling you to get only one bearded dragon (or at least separate them); a pet store owner has everything to gain to tell you otherwise, even if it's not in the best interest of the animal. Again, what I am saying is more for your parents than for you.
There are all kinds of instances of misinformed, misguided, or even purposely corrupt pet store owners and employees (unfortunately) who will try to sell you products that have been tested and tried again and again only to be found to be harmful to the pets that they are trying to sell to you (e.g., loose substrates such as sand, crushed walnuts, etc.; cheap, poorly-made, or incorrect UVB lights that have been known to cause eye damage or even blindness to certain animals; foods that cannot be easily digested by certain animals; etc.). Again, by no means are all pet store owners or employees like this, or mean to be like this (some of them are genuinely not taught the proper care), and I am sure that there are cases where the pet store owner and employees generally care about the animals well-being before selling them to customers. But again, have your parents consider whether or not they want to be listening to a site with hundreds and hundreds of caretakers with years of actual experience all congregated to learn and teach one another, or from a small group of pet store employees who want to sell away the animal and really probably don't have to worry about whatever happens to it after it's been sold?
And the most compelling argument? Aren't these the same pet store employees who sold your father the two Chinese water dragons without telling him it was wrong to house two of them together; didn't know the animals' genders; allowed the two to fight; and sold your father the enclosure even though it was much, much too small for them? After that, why would he be so intent on believing them this time around?
Anyway, sorry about the long post. Again, it's a pretty heated topic around here. Glad you have the common sense to at least do research and try to convince your parents to do what's responsible. I only hope that things turn out well and that you are able to convince your parents to either only get one (and if this is the case, I hope that you are mature and responsible enough to make the sacrifice just this once) or to get two separate enclosures for two separated bearded dragons.
Please update us with your situation, if you are willing to share it with us. If you do decide to get one bearded dragon, or two separated bearded dragons, we would be more than happy to help you set-up the proper enclosures for them. Good luck.