Welcome to DCT
Yes, the Garmin or any other dashcam can be fitted there. You may find an issue getting power to it unless you have a source close enough. Also operating the cam may be awkward if that power source ian't switched on and off with the car ignition or through a phone app. I run a single cam in the rear but mine is wired to a switch I installed up front- not hard to do but not "plug and play" simple like using the same cam in front.
Here is my Garmin 55 mounted as a rear camera. My Toyota has a rear cigarette lighter plug that is switched off when key is out. I moved the plug to behind the body panel and zip tied to keep it tight so nobody could accidentally knock the power adapter loose by stuffing groceries or cargo into the trunk. If you already have two single-channel dashcams I can't think of a good reason to purchase a dual-channel camera for a front/rear setup. Maybe if you instead want a taxi cam for the cabin then sure, you need IR lights so buy a Viofo dual-channel IR camera or similar.