The add-on switch does require a neutral.
Neutrals are my enemy with this configuration. Ugh. So the white wire inbetween the primrary and slave switch – any chance of re-using this in some way for a neutral?
The white wire in your setup is always hot. Notice as power is coming to the light in your diagram that the white wire is hooked up directly to the fixture. Neutral is never sent to the switches in this setup. Do you have any other switches in these gang boxes that have neutrals? If they are in the same circuit you could tap into it, otherwise a relay is your only option.
Well – so on the Primary the 142999 should work with no Neutral. We established that. I guess then the other option is to just give up and close up the slave outlet. Sucks but I can’t set up a relay. It’s just the odd ball thing is there is an unused white wire coming into the primary switch from the light – its a three wire where only two wires (black and red) are used.
If that’s the case. Drop down the light fixture and reconfigure the wiring so you will have load, neutral and line at left box.
It worked!!! Thank you!!! 
There are some smart switches, like the Zooz, that work with existing switches for 3 way situations. The primary switch just needs to be changed and the remaining switches stay. You could consider this instead of closing up your Aux.
Well – what I was able to do here instead was I found a Neutral. From the light box to the primary light switch is a 3 wire that had an unused wire. I took down the light and wired the unused wire to the neutral (bound it with the one to the light). This gave me a neutral to the first and second switch. Got lucky I suppose – but taking the light down and looking inside (with the help here of course) was the trick.
So in a second part of my house I now have encountered the other (pain in the but) configuration below. So, why wouldn’t I just re-wire the wires at the base to give me a neutral at both primary and secondary switch locations, use the red wire as the traveller and use the GE switch plus GE add-on. Am I missing something? Seems like I have all the wires I need (no fishing) just need to re-route things from the base. What am I missing?
That wiring configuration will not work with GE switches. The reason is that at the master switch you’ll need line, load, neutral and traveler for a total of four wires (the add-on switch would be fine). You only have three wires at each switch location.
Ugh Yes you are right. What if I used the 14299 at the primary and re-worked a neutral to the secondary switch since that only needs two wires?
You can either add one wire between a switch and the other switch. Or you can add one wire between a switch and the light box. Find out which scenario is easiest.
Or more generally, if I have a 14299 and a GE Add-on – can you help me understand what my options are here? I’m thinking there might be one – just not sure. My problem is my house is so old fishing new wires is impossible.
There are other options but they involve different devices.
See the FAQ.
For this type of wiring you can use relays or switches by Cooper or Linear which do not require a traveler. Another option is Lutron Caseta, GE’s 14299 Dimmer, or Leviton’s ZSS10-G0Z switches that do not require neutrals.
Sorry I missed the most important part of your post you have a 14299. You’re existing wiring should work as you don’t need a neutral.
Line, load and traveler to the 14299, then the neutral and traveler to the add-on. You’ll need to access all three locations to change the wiring
Yeah…so I need to re-configure from the light to give me a Neutral for the add-on. Its hard to figure out though which switch is the primary 
You should own a voltage tester if you’re doing your own rewiring. Turn off breaker. Disconnect one switch wiring. Turn on breaker. Check for voltage at each switch. Label wires.
Yeah I do and I did – I figured this would be the only way. The strange thing is I still get readings from both switches. I removed both switches and tested every line.
What is throwing me off here is in the first ‘power to light’ example the load and the line both go to the primary switch – but in the second ‘power to light’ example the load goes to one switch and the line goes to the other.
Visualise it this way. Hot to switch, two travellers between switches, switched hot to light, neutral return to panel. All 3way work this way, there is just many different ways to wire it.
For your usage case: This is how you wire your ceiling box. The black wire to the slave switch is left disconnected, white to neutral, and red to traveller. The master switch gets white to line, black to load and red to traveller.
