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Sure. You can just add on a add-on low voltage bracket box.
Says it's for new work, but as long as you cut out enough to access the side of the existing box, you should be able to put it in securely. It uses clips to hold on and double sided tape to secure. The manufacturers support on HD says that it can be used on old work as well, with other reviewers having mixed luck. Depends on the thickness of the box you are clipping onto. Just make sure the existing box is sturdy.
It does state it's for plastic and not metal boxes, as metal boxes are thinner walled. You could try to drill a hole and use plastic nut/bolt.
Update: it wasn't immediately obvious from the way you presented your question that this is something you're already aware of and don't like.
Nevertheless I'm still going to leave this answer up for others who may not be averse to using this solution.
No, a pair of single gang boxes or low-voltage rings (or combination thereof) side by side will not fit a standard 2-gang faceplate.
The thing you're looking for is a Dual Voltage Box
This example is a "New Work" box, but they exist in "Old Work" versions too.
The overall shape & size is identical to a standard 2-gang box (unlike what you'd get if you put a pair of single gangs side by side), and you get complete isolation between the line voltage and low voltage sections.
"No Volume Limit" Advantage of Open-back Boxes
The open-backed boxes have the advantage that they don't limit the termination area's volume to the size of the box. Here are a few scenarios where having more volume than what a box provides is important:
- In the case where the structured cable will be run directly to a device in the room (think speakers), the distance of cable needed in the room is spooled in the wall at rough-in and stays there until it is needed (you can cover it with a blank plate to make it tidy until you really need it). Because the slack doesn't protrude from the wall, it does not interfere with or get damaged by installation of sheet rock or other wall material.
- Where you have a high-density of terminations at the wall plate (think 6x keystone jacks in a single-gang box) the minimum bend radius requirement of cat5/5e/6 cannot be honored in a closed-back box. The slack required to make the terminations needs to go behind the wall plate which will result in a very tight coil of cable in the box at best, or will outright not fit at worst. Open-backed boxes solve this problem because the wall cavity can be used to house the slack.
- Using an in-wall active termination device like this might require more volume than a closed-back box. The box would need to accomodate the bulk of the electronics plus the slack required for termination without violating the bend radius requirements of the signal cable.
Note that on exterior walls vapor barrier and insulation will limit the volume available to you, but likely far less so than a closed-back box.
Vapour Barrier
Where I live (Canada) it is customary and code-acceptable to use "vapor barrier boxes" (pre-formed poly cups shown here) behind all electrical boxes on exterior walls whether open- or closed-back. A regular closed-back box does not provide its own vapor barrier any more than an open-backed box.
This method comes from the building code decree that "thou shalt not compromise vapor barrier with wiring." The vapor barrier boxes are taped to the wall's vapor barrier, and wires are puttied to the vapor barrier boxes where they penetrate.
Insulation
I have seen no issues with the combination of open-backed boxes and fiberglass or mineral-wool insulation. Assuming a proper vapor barrier is installed around the boxes, there should also be no issue with spray-type insulation.
The main benefit is they are made to accommodate smurf tube with the large round holes. So, if you're using smurf tub to run the cables, you'll likely prefer the open back boxes.
Otherwise, it really doesn't matter.