Most of this is pretty simple if you are familiar with working with paths. However, there are a few tricks to the alignment and combining that you need to know in order to get the right results. Open-ended paths and paths for which the wrong combining mode are selected for will not produce the proper results. As part of the process, Photoshop will first close the ends of any open paths and then combine the shapes. This may lead to some unexpected results in practice -- though they are logical results.
Alignment will have to be perfect for the shapes to combine, unless the shapes overlap. If you don't overlap and alignment isn't perfect, you'll still get one shape when you combine, but it may have some faults, such as a very thin space between the halves (this may look like a center line). To be sure you have everything perfectly aligned, you may need to view at 1600% and use the keyboard arrows for fine adjustments.
It might just be best to close all your paths and create an overlap. For example, if making a heart quickly, instead of worrying too much about perfect horizontal alignment, you can create an overlap that will make sure the shapes combine. This overlap will extend beyond the center line. See the example in Figure 5.
This probably wouldn't be necessary for such a simple shape, but more complex shapes might require more complex handling.
If you highlight both 'halves' you can combine the two as a single shape, exactly as in the steps. However, the end result will cover over any slight misalignment, and assure the shape is whole.
If you choose to overlap, it may be best to take an extra step. Instead of just guessing where the overlap should occur, duplicate the original, place it to use as a rough outline, and then use the outline to figure out where the overlap really needs to occur. Make the overlap changes to the original half only. Once you are done, delete the duplicate that you were using for the outline, and then duplicate the original once again, repeating the steps for placing it. This will make sure both halves have the overlap and that they are identical, which is essential to getting perfectly balanced results when combining the halves.