There has to be a junction box somewhere in the middle but I have no idea where.
This is the upstairs box. At first glance I assumed the two wire went to the light and the three wire went to the second switch. The thing that confused me is this has a lighted switch since it’s for an outside light. I disconnect all wires and measured between ground and each wire. None were hot.
Downstairs:
This is where the fun begins… with all wires connected and metering to ground, the black is hot. Red and white are dead. With the power back off I twisted the upstairs black and white upstairs and measure 1 ohm between white and red downstairs.
Edit: Ideally Blue goes in the upstairs location.
Edit2: Aux is fine in downstair locations.
Edit 3:
My hypothesis, maybe @Bry or @harjms can confirm. Downstairs connect black to white and black to aux neutral port. Connect aux traveler to red. Upstairs connect black from 3 wire to line, white to neutral, red to traveler, black from 2 wire to load. My only concern is that neutral configs usually sent a neutral to the aux and not a hot.
I’m not sure. You would think so, but there were indicator lights that worked without them. I don’t have experience with them though. Us a meter and test between that hot black and the white in the downstairs box and see if you get 120VAC.
The problem is you only have a 3-wire going into that box. If the hot starts there and the white is the neutral, then you’re missing a traveler to the other switch (you need two).
Sorry i meant the white on the second floor. The white and red in the downstairs box are the traveler lines. As far as i can tell the white and red from the downstairs end up black and red on the second floor. The upstairs white must be connected elsewhere in a junction box to provide the neutral.
Well, that certainly looks consistent with what you’re described so far. I suppose you can do a couple more continuity tests to confirm.
The problem is, as you mentioned in your original post, is that there isn’t a way to send the neutral back to the 1st floor if you want to put an Aux there, as you only have 2 conductors to work with, if you can’t find that mystery box. (One for the hot and the other for the traveler.)
It won’t work the other way around either (with the Aux on the 2nd floor). Either way, you need 3 conductors between the switches.
Since this is an outside light, is it an on/off? If that’s the case, you can do 2 Blues w/o binding. Make the 2nd floor the “real” switch and the 1st floor a scene controller. Instead of binding them, you can control the 2nd floor and sync the 1st floor with automations. If you do that, send the hot from the 1st fl to the 2nd floor over the red. Send the neutral from the 2nd fl to the 1st floor over the black/white.
I know @harjms is looking at this tonite, so he may have some thoughts.
