You would need to record as well on the Fenix (discard rather than saving at the end to avoid duplicates).
I do like the watch vibration alerts on mountain descents when there is too much wind noise to be confident of hearing the head unit beeps.
The just released Varia 515 includes audio alerts through headphones and BT pairing with your phone. Although if the situation is sketchy enough that you are worried about wind noise and not looking down, you probably shouldn’t be wearing headphones.
I think the watch will (eventually) time out if you don’t start recording.
The watch does need to be actively recording an activity for the Varia to show up as an alert. I tried it last night and it worked as long as you are recording something. So I think I will setup a profile that doesn’t use much battery and doesn’t pair to anything other than the Varia so I can get alerts on the Fenix. Looks like I can make it work and appreciate the assistance!
Does the watch have a “indoor” profile like the headunits do that keeps gps disabled? If so that would save even more battery.
It sure does. I’m going to try that next time I head out for a ride. I will be super happy if this works and wouldn’t see a need (for me) to upgrade to the new version. 
I vaguely remember a recent update that turned the headlights off for the indoor profile (although I could be confusing it with Edge updates).
I’d suggest creating a custom profile by copying the existing Bike profile, then modifying the settings there to reduce battery consumption (eg. selecting Ultratrac).
I really hope they add the low brightness mode to the RTL510 - it’s far too bright most of the time.
@cwiggum yes it does turn them off while riding indoor as I use my 530 indoor to record along with TR.
i have my light always on standby mode and when you start up an indoor activity the light will flash for one second them go off completely.
Bet you can turn the light on manually if indoor is turning it off.
Never tried with indoor workout. Doubt it will turn on due to the activity setting.
Yes. Anyone within range can receive the information given their head unit is capable of displaying it.
I believe @gpl says it is ill-advised.
Yes, you can share an Garmin RTL515 with a partner: Garmin Varia Radar: Share it with a Friend! - YouTube
Edit: yes, it’s possible.
Should you? Well that’s a whole nuther question…
Not a good idea in my mind: not only will the distance measurement be out of whack, but you’d have to be riding in a peloton for this to make sense (and so that you are within range).
Can you elaborate on this? In a scenario where only one rider in a group of 2 or 3 has the Varia, why wouldn’t you all link to that? Some information is better than none.
Of course you’d have to stay in range - the head unit will tell you when you drop the link. IIRC the range was 20-30 feet when we played with this.
Sorry, I wasn’t very clear when I posted: I wasn’t referring to the range as a problem, I was thinking of bunch riding as an issue. Like @gpl wrote, unless the person with the radar is always on rear guard duty, the radar beam may be obstructed by the other riders, and people who piggy back on the signal may be lulled into a false sense of security.
I have been running a Varia for two years. I can’t image myself riding with out it anymore. I even forgot my mount and it still worked through my Jersey pocket!!
I agree with. GP lama. People behind you can block the signal. But when I ride with people I still find it effective enough. When I get the red warning, I look behind and see a huge truck. I worn people or just pull off the rode if Im riding alone.
Its not going to keep you from getting hit. But the early detection lets you at least get of the road if you choose too.
Its better than nothing in my opinion.
Thanks for the complete response, but I disagree.
You’ve made an assumption about the size of the ‘group’ and how they ride. The question here about a riding partner. that sounds like one (or maybe) two extra riders. Not hard to manage that group. I can imagine novice casual rides where there is somebody who always rides sweep.
While there are times when it is a bad idea, that doesn’t mean that it should never be done.
I don’t think anyone should rely on the radar for their complete situational awareness. If you move over without checking, because the radar doesn’t show anything, then you are asking for trouble.
Doesn’t work. Tested with a 1030, you can configure the head unit to be paired with multiple devices, but it will only connect to one at a time.