So, I'm on my friends Realm and we're trying to make a Nether hub, which we did in a previous Realm about a year ago (hers expired and didn't get a new one). She built up a portal that's 5 across, and 6 up and lighted it. She jumped through and then came back, so she could start getting coordinates of my portal so she could make one in the Nether that would make you go directly from there to my portal in the Overworld. Now, here's the problem: She came out of the portal, and ended up in a new place. I went through my own portal, then back, and ended up coming out of the same portal she went through coming back. She had to go, so I destroyed the portal that we kept coming out of. Then, I went through the big portal she made and then back, and it spawned another portal again. Both portals are close to the big portal she made, but we're still confused on why it keeps spawning new portals and not letting us come back through the one we already made.
Be sure to note, in addition to Kevin's answer, that you might want to test exactly your situation's ratio. If you are playing on PC or console, and if you are playing on console which one, and which world size. All of those things allow for radically different ratios. You can test it yourself in a creative world with the same map properties...just build a few dozen portals and experiment a little with them.
An additional thing to note is that portals will seek out the nearest available portal if you build them too close to each other, with exceptions. One with careful placement and activation orders can link portals in complex but specific ways.
PC example of the concepts:
In one of my worlds (Xbox 360) I have an underground pyramid. At the top of the pyramid is a portal which anyone will find on their way down from the overworld. If they enter this portal they go to the nether hub, if they enter the portal in the nether hub they return to this top portal.
I have this or a similar design (one of Mumbo's probably) nearby in my pyramid:
This hidden portal actually turns /off/ as you teleport. It takes you to a secret room BELOW the nether hub portal. Taking the portal out of there...well the portal you took to get in is hidden/turned off so it "doesn't exist" and the game will want to create a new exit portal for me, but I purposefully placed it close enough to the "normal" portal that instead of creating a new portal close to where the "hidden" one is it just sends you to the former.
Oh this isn't always clear but only the x and z, or the horizontal planes, matter. They can be any /height/ difference and still link up, but once you move them apart from each other horizontally in any direction that is what changes the linking.
Using these techniques one can create a very intricate hidden travel network that hides behind the "surface" travel network. Very fun stuff for being an admin in a game without giving myself admin powers...I just build the world itself to be my "admin powers". :)
Press F3. You'll get a big debug screen with a lot of information. On the left side, you'll see things labeled "X" and "Y" and "Z". These are your coordinates. They describe where you are in the Minecraft world.
As I'm sure you are aware, the Nether multiplies distance. Specifically, if you are at coordinates (x, y, z) in the Overworld and build a portal, you will arrive at least close to (x/8, y, z/8) in the Nether. How close you arrive depends on a number of factors, particularly what blocks are in the landing zone. This works in reverse when you take a portal from the Nether to the Overworld.
To make this work perfectly, you need to ensure the Overworld and Nether portals are positioned very close to where they should be according to the above formula. So decide which portal you want to move, note the coordinates of the other one, destroy the one you're moving, and rebuild it at the correct location. For example, if you're moving the Nether portal, and the Overworld portal is at (64, 64, 64), you need to move the Nether portal to (8, 64, 8). Then it will link up properly with the Overworld portal. You will very likely need to build a tunnel in the Nether, since (8, 64, 8) is likely to be underground or otherwise inaccessible. If it were accessible, Minecraft would have put the Nether portal there to begin with.