I use an Op/Tech Double Sling.
It's as comfortable as any other strap I've ever used, and more comfortable than most. The modular OP/TECH system makes it easy to swap a camera from the harness to a single strap in mere seconds. It also allows the camera to be easily and quickly disconnected from the harness and stored in a case or backpack with only the uniloops still attached to the camera.
I typically have a gripped 5D Mark III with an EF 24-105mm f/4 L IS on the right side and a gripped 7D Mark II with an EF 70-200mm f/2.8 L IS II on the left. Sometimes I'll have a 50mm prime on the 5D Mark III and a 135mm prime on the 7D Mark II or the 135mm will be on the 5DIII and the 50mm will be on a 5D Mark II. I'm a large guy at 6'0" and a little over 300 pounds. It works fine for me.
I have the harness straps cinched up closer to my shoulders than what is shown in the product photos and use OP/TECH Extensions (Regular) with the OP/TECH Uni-Loop - System Connectors included with the harness on the right side. I have the front connector attached to the right camera lug (near the shutter button) and the rear connector attached to the strap lug (NOT the tripod receiver) on the base of the grip. The camera hangs just as shown in the photo except the "X" between the harness strap and the two connectors to the camera is higher up on my torso. I can even attach a full size speedlight (580EX II or YN685) to the camera's hot shoe and it still works well.
On the left side I have the front connector that slides on the harness attached to the strap lug next to the mode dial (on the top left of the camera) using an OP/TECH Uni-Loop XL. I also use an OP/TECH USA Extension (X-Long) attached to the rear sliding connector as a non-load bearing 'safety strap' attached to the strap lug (NOT the tripod receiver) via an XL uniloop on the bottom of the grip. If for some reason the single load bearing connection on that side of the harness were to fail the 'safety strap' would catch the camera and lens without letting it fall to the ground.
To date I have never had an OP/TECH connector fail. I have had a couple of occasions where I failed to fully click in a quick disconnect and having a redundant connector on both sides prevented a dropped camera and lens!
I sometimes use the harness with a single camera attached on the right side. To keep the harness centered with no weight on the left side I cinch the X-Long extension attached to the rear sliding connector on the left side all the way up, loop it under my belt, and click it back to the front sliding connector.
I also sometimes use the Canon CPS logoed version (provided by the Canon Professional Services program when first joining at the Gold or Platinum level) of the OP/TECH Utility Strap - 3/8-Inch (Black) as a more traditional neck/shoulder strap. I use the pad from the Utility Strap connected to the left side of the camera via an X-Long extension cinched all the way up (the one I had left from the pair I bought to use one as a safety strap when carrying a camera on the left side of the harness) and a uniloop already attached to the left camera lug. I use the regular extension strap, let out enough to match the length of the X-Long extension on the left side, already attached to the right camera lug via a regular uniloop to connect to the right side of the Utility Strap pad.
It's actually a lot simpler to do than it is to explain here. You just need to pay attention to the male/female ends on the uniloops and extensions. In general the male ends are on the left end of each piece and always point from right to left and click into the female ends on the right end of the next piece. Once the uniloops are attached to each camera with the correct gender on each side I can swap cameras and straps almost effortlessly in just a few seconds.