Symmetrical objects: how to edit them?

Astute has a free plugin that makes live symmetry easy and fast. It breaks the dynamic corners unfortunately though.

Incredible, that the biggest vector software in the world, STILL doesn't have this feature. Wake up Adobe!

Shepard,

I (mis)read your first question to mean that you wished to work with the actual whole shape as a closed path.

It is quite possible to create a symmetrical drawing live with the help of Illy, with the inbuilt tools, without any external plugins, if you just start in the right way. Obviously, it is much better to see the whole appearance while you create it.

Depending on the number of reflexion/rotation axes, there are different ways.

For a simple one to two mirroring such as a heart, you may see the reflection live after each Anchor Point if you start out as follows (for Pen Tool with vertical reflection to the right, similar for other directions):

1) Create the first two Anchor Points of your basic path;
2) Effect>Distort&Transform>Transform>ReflectX with 1 copy and the middle right side Reference Point chosen;
3) With the Pen Tool ClickDrag (or Click) anew from the second Anchor Point and go on;

If you wish, you may:

4) Object>Expand Appearance and Ctrl/Cmd+J to join the half paths.

Ctrl/Cmd+Z Undo is your friend: you can just go back and redo while drawing the basic path.

For multiple reflexion/rotation axes and plugins, you may have a look at solutions in this thread, including the links to external solutions below:
https://forums.adobe.com/thread/2284752

Or you can have a look at the MadPattern templates,
http://madpattern.com/

Or you can use the MirrorMe plugin from Astute Graphics,
http://www.astutegraphics.com/software/mirrorme/

There are several ways to do symmetrical in illustrator, one of the easiest is with the help of the transform panel via appearance panel, see in my picture attached the basic parameters.

When finished editing your shape, just duplicate it and expand the appearance for final toches if needed, keep the original for other editions.

Shepard,

Just returning after leaving right after posting.

I can see that I ought to have included Handles in my such as these list.

But luckily you got it from K.

Shepard,

Given your closed symmetrical shape, you can use good old ways to work with corresponding Anchor Points and/or segments, such as these, each time time starting out with the object deselected and being able to see what you are doing while doing it:

To move up/down, you can ClickDrag over a pair/set of corresponding Anchor Points/segments with the Direct Selection Tool and ShiftClickDrag up/down or use the up/down Arrow key.

To move apart/together, you can ClickDrag over a pair/set of corresponding Anchor Points/segments with the Direct Selection Tool and then switch to the Scale Tool and ShiftClickDrag either out/in.