If you have two girls, staying in one room of your house you can use the bookcases of the kids as a temporary divider of their two cubicles, so each girl can have some degree of privacy in her cubicle. This is necessary for you to do until you can have an additional room made to which one of the girls can move later.
As the girls grow up you can expect them to ask for an individual room each. When they are fast growing into their teenage years they likely would want their own room to decorate, to arrange things to their individual liking, and other things on which each girl might have individual preference.
Their wood bookcases now can serve the purpose of separating each girl as if the resulting two cubicles were two individual rooms. Place the
next to each other, one of them facing towards the direction of one of the girls' cubicles, the other facing in the opposite direction.
You can expect each girl to be quite fussy on how she arranges her things, knowing the nature of the female of the species. Just in the matter of decorating their separate cubicles, you cannot expect the girls to think in the same way. If one girl for example thinks she prefers pink as her color, the other girl will always contradict and might opt for another color to be the prevailing color in her cubicle.
You may have observed these womanly habits with your wife and her sister. Seldom do they agree in one color or design of their clothes. By nature, women do not like to wear the same clothes as other women, not even among sisters. Each would like to have something different from one another.
You will notice this early with your daughters. They will start behaving just like any woman; they would want to differ from one another. Your wife of course is the first to point out to each girl that they should not have the same color or design of their clothes, and in many other things in the possession of each girl.
As the girls grow up their individuality will develop in the same manner that your wife developed her own different preferences for anything from her sister before. Your daughters anyway have the same genes as your wife, so they will each behave differently, especially in the choice of clothes. You will be overruled every time when it comes to the choice of the girls’ clothes, so when they go shopping for these things, better stay home. If you have to drive them to the mall for this, stay put in one of those coffee shops there, and enjoy your cappuccino while they take their time fussing over the clothes in the women’s store.
You say that you do not understand why women behave this way. Maybe you have not really known your wife all these years. Well, if your wife was one of those sisters their father before separated in two cubicles of only one room, it will be good for you to start understanding the ways of women now. Your little daughters will be women not so long from now.