This is a very common problem. You must understand that the girls will rarely compete with other girls in same bar. It is an unwritten law, the Code of the BGs. When you sit with one, then you are her customer and none of the others will go with you. Whoever pulls you into the bar gets you. You are her customer until she lets go her claim on you. So if you fancy another girl, you have to be very firm, and tell her to go away, you don't want her. You have to tell her in no uncertain terms. Better yet, tell her that you don't want to go with her, but that you fancy her friend. If she is very clear that she has no chance with you, then she may turn you over to her friend and it is a good way to get an introduction. But her friend will not do it without her OK.
Once you actually go with a girl, then you will not be able to get a girl from that bar to go with you until she relinquishes her claim on you. If you only had a drink with a girl, then left the bar, come back 2 nights later, then if the girl you drank with sees you and runs to you, you must turn her away or you will have the same problem. If she sees you and doesn't latch onto you when you walk in, you are free game. If she doesn't see you or is with another customer, then go directly to the one you want and buy her a drink. If she accepts the drink and sits with you, then you will probably be OK, but if need be, you must be clear that the first girl is rejected and has no chance with you.
To avoid this problem, I look over all the girls first and am very choosy about who I sit down with.
The only exception to the rule is if you establish Butterfly status in a bar. On my second tour, there was a bar in Seadragon where I went most nights. I made it clear that I was a butterfly, and never went with the same girl two nights in a row. There was a girl in the bar who was a friend, who I sometimes BFed to go to dinner or out drinking, but never for sex. I would go and sit with her, and look over the field. When I fancied one, I would go buy her a drink or have my friend ask her to join us. After not long, it was recognized by all the girls, that I was a free agent and let no BG have a claim on me. I went with many of the girls in that bar with never any hassles. On any given night, I took my pick. Some I went with 4 or 5 times, but never on consecutive nights.
JayBee
P.S. The girls don't care what you think or who your "target" is. To get what you want, sometimes it will require to hurt one girls's feelings a little bit, but, believe me, it is not heartbreak, because they don't feel anything for you. It is just normal rejection, just loss of face and loss of potential payday, no feelings of the heart involved at all.