You can join the Arsenal membership as a Red member, normally tickets are available to Red members once higher categories are sold out and you shouldn't have problems for a match against one of the lower half teams, just check beforehand with the club that you can get 2 tickets with your membership. Depending on the draw, FA cup early round tickets usually go on general sale. Check the Arsenal web site for more details under Membership. They also have a scheme where members/season ticket holder who cannot attend a game can sell their tickets to other members