See my edit- it worked great for a little while then crapped out. Back to the drawing board…
Bummer
If the only answer is to buy two, full-blown (not inexpensive) switches/dimmers to make this happen–I’d strongly encourage @Eric_Inovelli to push for some sort of firmware update to let the two keep their LEDs in sync. That LED is the one thing that would somewhat help justify paying the extra money vs. the GE Aux Switch.
What hub are you guys using? This should be possible with ST and Hubitat. I was actually going to write something up shortly.
Basically, it will require an automation to be setup and it won’t show the status instantly, but it should be very close with Hubitat as the cloud isn’t there to slow things down.
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I use HomeSeer. In HomeSeer it’s easy, there’s a function called Linked Devices. Whenever one dimmer changes, it will send a control command to the other dimmer to change it. I’d assume Hubitat has much the same.
And @Eric_Inovelli… HomeSeer dimmers can 3way with Z-Wave associations… perhaps something to fix in a future firmware update? 
I played with it a bit on two Red series dimmers, and the best result I got was associating dimmer 1 to dimmer 2 with groups 2 and 4, then associate dimmer 2 to dimmer 1 with group 3. That worked reliably, but in one direction (from one dimmer to the other) the dim command wouldn’t take effect until I let go of holding the paddle. From the other dimmer, dimming worked perfectly and live on both dimmers.
It seemed like what was happening was they were getting out of sync and trying to update each other, thus creating a loop. I’m sure there’s some firmware tweak that would fix that, maybe ignore dim commands from associated devices while the paddle is being held down or something… hoping this can be fixed in a future firmware because I’m hoping to use two Red dimmers in this exact application…