In the 1940's the Nine Old Men (then known as the Disney Animation Board) it was decided that it would be more effective that the system of who animated what be changed from animators specialising in particular characters to animators taking on entire sequences.
The reasoning was twofold. One of course was the sheer 'logisitics' of coordinating scenes where varios animators had to animate characters in the same scene. Since each character needs to be carefully controled so there is harmony and balance between them then a decision always had to be made who would be the lead off animator to which all the others would follow. A situation not the liking of all the animators.
The second was if an animator had an entire sequence to himself he could feel and control completely the emotions and the movements of all the characters in each scene and so better garanteeing a coherent and fulfilling result.
Although this sounds logical I wonder if it was just as much these reasons as the Nine Old Men's growing individuality and their most probable inability to maintain the kind of communication and permissiveness that sharing scenes would allow them.
I say this because in the recent 'Golden Age' animators reverted back to specialising in individual characters which when you think about it is really the best way for an animator to fully take advantage of all a character's possibilities and clearly control his or hers performance during an entire film, more closely to the live action experience.