Hey, I figured it out.
I think I model like most people do, and model one side of an object then mirror it.
Well, (assuming everyone else does it this way) this is how to use the mirror function.
When you mirror an object, you need to do a few more steps.
After you mirror your object, you actually get two separate objects. We now need to make this one object. To do this, select both objects (your original and mirrored), click the BUILD tab, and then JOIN SOLIDS.
Now you have one whole object. But we aren't done yet. You might notice you have a big seam going down the middle of your model. This is due to the points overlap one another. To fix this, do the following.
Select all the points going down the middle of your model. Click the EDIT tab, then click on MERGE POINTS. (I usually just press shift+l (make sure you have CAPS LOCK off)).
The box will appear and it will give you an option to change the number. This number is the distance between the points to merge. If your number is too high, you might merge some points you don't want to. I usually do .025, but play around with it to understand it.
Now you have your object mirror, and you got rid of your seam.
To enable the NEW MIRROR function, simply click on BUILD tab, and hover over MIRRORED. Select the CONVERT TO MIRROR button. And there you have it. Every thing you do on one side of the model will be mimicked on the other side.
I should have prefaced this 1st, but I just discovered the new mirror function midway through making my model. So I'm not sure how you should start it, but I'm sure you can do the same thing with two polygons. Anyway, If you didn't know about this already. You do now.