There are plenty of dual mounts on eBay/Amazon that connect to an existing single mount.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

https://www.amazon.com/...;ref=nb_sb_ss_i_1_10

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

Something like Titanflexer linked to is your best bet, but shop around; there are a bunch of folks selling essentially the same thing with pretty wide price variance.

You'll probably be ok if you're just riding on the road, but keep a close eye on things. I've broken a couple of mounts over the last several months, as I've been doing product testing that involves doubling up on computers and lights and such on various mounting devices. Be especially vigilant with anything that screws together. 2 different mounts that are "permanently" screwed together came apart on me after a couple years of solid service once I started hanging too much stuff on them.

I would absolutely tether things to the bike just so you don't lose everything if the mount breaks.

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Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/

Can't tell what that Garmin mount is made of.
If not aluminum, probably go for something else.
K-edge makes nice ones that are combo, edge on top, gopro mount on bottom.
Then get a sturdy double like titanflexer posted.