Good Morning Katdiaz-
In the animal kingdom (yes, we are all animals) there is an instinct to be dominant in order to perpetuate the species with the strongest genes. As humans we interfere with the normal sequence of events by intervening in the establishment of the pecking order to protect a younger or smaller child from an older, stronger child. We, obviously, use our adult size, intelligence and otherwise to accomplish this. This "artificial" intervention is not abnormal, wrong, or necessairly deleterious to the development of either the older or younger child. It's just the way our species acts normally.
This does not mean that the children will not attempt to exercise their innate needs to establish dominance. Indeed, they should. It's all part of their growth where they hone the skills necessary to cope with their environment.
That said, as the age range between children grows, the things they have in common decrease as do the opportunities to be together. A 12 year old child will have little in common with a 2 year old, other than perhaps to babysit. In that case they are the authority figure and not in competition.
So, to try to address your question directly: Since there is nothing bad or wrong about children trying to do what their genes direct, the question evolves to other issues. Your (the adult) emotional situation, your financial position, your living situation, your career situation - you get the idea. Since it is your ultimate responsibility to provide the best care you can to your children (regardless of age), you are best off planning your family with these practical issues firmly in mind.
As an aside, there is exactly 5 years between my two (now grown). The younger always perceived a competition while the older had no such feelings. Prespective is really something isn't it?
Oh, my mother always said that you don't want two children in diapers at the same time.
Hope this helps.