quick question: edit paths simultaneously

andreaf23600721  wrote I want to curve the middle of the bottle exactly the same.

Hi Andrea,

• The easiest way is to use Reflect as an Effect. Draw half of your shape and align the anchor points on the open side with the Direct Selection tool
  Deselect, then select the path with your Selection tool.
  Go to Effect > Distort & Transform > Transform...
  
• Reflect X. In the widget, choose any of the three boxes on the right edge for this example. Enable Preview to make sure you have the right settings.
  
• When you edit the curve on the real side, the side that is a transformation effect will reflect the change in real time.
  
• Remember to copy the bottle and expand the copy when you think you are finished.

Photo credit to davescm​ in his weekly Photoshop Challenge from Week 56:
Something for the weekend - Part 56 - Hitting the bottle!

Actually, you're really close to answering your own question. If you have 2 paths, you could add an anchor point to each, use the Direct Selection Tool to select them both, drag them at the same time to make 2 angles. Now you have 2 corners and can use the Live Corners Widget to make the curves at the same time. Here's more info on the Live Corners Widget How to work with Live Corners in Illustrator

Andrea,

Often it is necessary/better to be able to see both mirrored/identical halves (to be) while drawing.

For a simple one to two mirroring, 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.

I'd recommend drawing or modifying curve on half of the bottle, then duplicate and reflect that shape, finally unite both of the shapes together using pathfinder tool.