I think you can try and plan it a certain way, hoping your "pairs' will get along. However, I think it comes down to personalities and parenting. I wanted my kids close, in groups of 2. The oldest 2 are 14mo. apart and both girls. About 2 years later, I had another girl (#3) and #4 are 13mo. apart. Now #5 is going to be a little over 2 years younger than #4. The oldest 2 play together well, and the younger 2 play together well when the older ones are at school. They all fight at times too.
I think what mistake parents can make is having fixed expectations (ie:you are the oldest, you should be the neatest, most punctual....set the example, high acheiver....smartest) and then comparing one to the other. If your child picks up on that, they grow to resent the one you "favor". I think if you're careful to let them be how God made them and expect the same behavior and responsiblity wise, there will be less resentment. (of course I have a long way to go yet!) I just don't ever want to see my teens, or grown children (hating one another-that would break my heart)