I think you're overthinking this. Personally, I don't care if they're in sync because there is absolutely nothing to be gained. My fenix thinks I'm a cycling imbecile and my edge has no idea what running is. Doesn't affect me one way or another. As far as running both at races, I like having the redundancy of independent units. I often screw up and forget to hit the lap button on my fenix or hit it too many times and next think you know I have an obscenely long T2. I usually just use the edge file and delete the other one.

So, you are basically looking for a solution that will limit the functionality of your bike computer. It doesn't actually make sense. If you want to keep all the data on one device you should connect your bike sensors to your Fenix, do the bike leg with both Fenix and 520 and then not save the session with the bike computer. I don't think you'd find an external monitor type for your Fenix, since it doesn't broadcast any data.

Misuse of consumer computing devices in ways never conceived by the manufacturer, let alone supported, leads to pain. Pain leads to anger. Anger leads to hate. Hate leads to suffering. Suffering leads to the Dark Side. Also known as Linux. You can do anything in Linux. Anything. Be warned though, for it is a long and uncertain journey.

Do you ever get concerned that someone may steal it from the transition area?

I might be overthinking this, but I'd really like Garmin connect to act as a syncing hub for all my devices. I want everything to work together as I think it should.

As for restricting use of my 520, that's not Really it. Because the denizens and 520 don't really work perfectly together, I'd be interested in a device that worked as a reflection of the fenix.

When I imagine all this stuff working in my head I compare it to my iPhone where stuff from my work computer and home computer just shows up and works together.

1. Yes records won't automatically synch back and forth between the devices.
2. Your FTP should be set in Garmin connect and then pushed to the devices. If you set it on one of the devices, it will not push to the other device.
3. What you are asking for doesn't exist (as far as watch mimicking your bike bike computer). There is no market for this and sounds really silly. Who looks at their watch on the bike anyway? I only look at my 520 during the bike. And I always use my 520 file to keep for later. My 920xt bike file always gets deleted.

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Perhaps on not explaining myself clearly. garmin connect is not reliable in pushing accurate numbers to any device. I've changed HR and ftp zones in the app on my phone to find it not changed when I log into it via the web browser.

And since GC is unreliable, I'd be interested in a master device like a watch to have little slave devices that run off it. I don't look at my watch while biking, that's why something on my handlebars that reflects what my watch says could be useful.

Do you use an iPhone? More specifically do you use the Health app on the iPhone? I was having a similar problem with my Strava re: weight not setting and I found out it was because I'd synced my weight to the Health app, so that was overriding whatever I'd entered on the Strava app.

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Disclaimer: Not a triathlete anymore, just a fish

I think DC rainmaker discuses part of this issue on one of his blog posts, or maybe the podcast - that garmin have said they are going to implement the duplication thing in connect - so if you upload two activities that are the same from two devices, it'll delete the duplicate.

Its only an issue at races - the rest of the time I use my 820 for cycling and my 920 for everything else.

A long while back I got a gift of a 920xt, I already had a 910xt I'd been using for a few years.
The 910xt was fitted with a quick release so I relegated that to use as a bike computer and now leave the 920xt on my wrist. So, essentially the same situation as you have with your Fenix and 520.

My 920xt is my device of record. All training sessions and races are recorded on it.
The 910xt is purely a reference unit so I don't have to look at my wrist.
I will typically record cycling activities on the 910xt too, but it's not synched to CG.
I only upload an activity in the rare circumstance where I've had a problem with the 920xt (dead battery, accidental press of stop button or missed lap button press in transition).

This is a very simple way to do things. I am not guaranteed that the data on the 910xt is exactly the same as the 920xt. Since I don't keep it synchronised I don't know that my weight or HR zones are identical but how important is that really? How often do HR zones change significantly? Does it matter to your bike data if your weight changes by a few kilogrames? I suppose if I used power on the road I might like to have updated FTP but even that is unlikely to be critical for most purposes and would be easily updated from time to time to account for changes if it is important to you.
Once you get into master and slave setups you may add a tiny bit of convenience but you also add system complexity and failure modes. Half the benefit of my setup is that I have redundancy. If the 910xt was a simple slave acting purely as a mirror of the 920xt display then this redundancy would be lost.

This seems like the wrong solution to a tiny problem. There are a number of things I'd like to see improved with CG and Garmin devices but this isn't one of them.
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