I'm placing two end point anchors on top of each other and joining them into a single smooth anchor
What I feel works better is a simple menu shortcut..
If the anchors are already on top of one another....
- Select the anchors (click-drag)
- Hit Command/Ctrl+Option/Alt+Shift+J
You will be presented with a popup asking if you want a smooth point or a corner point, just select Smooth.
This shortcut is for Average then Join. By averaging the two points first (both vertically and horizontally), the anchors are precisely on top of one another. Therefore when you Join, AI assumes you want 1 anchor, not 2, and allows you to choose (asks you) whether that anchor is a corner point or a smooth point.
If there's some distance between the two anchors, you want to do this with an extra step...
- select both anchors
- Hit Command/Ctrl+Option/Alt+J
- Then Command/Ctrl+Option/Alt+Shift+J
The first shortcut simply averages the 2 anchors to the same location - this allows the second shortcut to ask you if you want a smooth or corner point when joining them.
In short, you don't really need an action. It merely takes some getting used to the spiderweb like shortcut. You'll always be presented with the smooth/corner pop up. I'm unaware of any way around that dialog window. But having the popup right there in front of you is much easier than hunting/peaking a control bar button, at least for me. And 90% of the time you can merely tap the Return/enter key to dismiss the pop up because it'll be set on whatever your last choice was.
This shortcut is a default shortcut and is buried in the keyboard shortcut menu somewhere. I'm not sure where, but I know it's there in case you want to alter it to a more convenient shortcut. I, personally, tend to not alter default shortcuts if it can be avoided so there's no learning curve when new versions are released.