Personally I use a Black Rapid Double Breathe and usually carry two gripped full frame bodies, two pro lenses (usually 24-70 + 70-200 (occasionly with a 2x)) along with flashes. No problem. It also comes apart into two seperate single harnesses if you only have one body with you (you can use a double harness with one camera... but it sits lop sided and you look like a twit).

That's my personal preference.

Whatever you spend your money on I recommend doing some research. I switched from two shoulder straps to a harness after spending 12 hours on my feet at a music festival and by the end was in pain. I've never looked back since making the switch. But these are the points I would consider. For any system.

1. Weight distribution/padding. Two camera bodies get heavy, not neccessarily straight away but after time, and you do want to get one padded to stop digging int your shoulders.
2. Asjustability. Can you adjust the strap to make it comfortable to use/carry.
3. Build Quality. eBay (for example) is filled with off market straps very similar, usually from China. Due to unbranding finding a manufacturer can be hard. You need one which isn't going to fail, either leaving you with cameras and no way to carry them, or worse.. they crash on the ground and you have an insurance claim to make. Things to be mindful of are any points of failure. Clips and moving parts. The material the straps are made from too. Don't forget a lot of weight/money is going to be relying on this strap. Don't scrimp and get a cheaper option which looks the same. Hint: If it has a manufacturer guarantee (the longer the better) then it's a good sign. The company has faith in their product., as they wouldn't offer it, if everyone ended up claiming on them
4. Can you carry an additional bag and be comfortable. Seriously. You may not be transporting your gear on the straps, and may have extra gear with you. Can you keep use of your strap whilst still having access to the bags? (I can use either a backpack and/or a belt mounted lens pouch with mine).
5. Does it have guards. I didn't know about this but on mine I have plastic adjustable guards I cna use to prevent the distance the camera can freely move on the straps by its self. These little things are incredibly useful.
6. Reviews. Look on photography blogs etc, see if the product has been reviewed and especially stress tested, in order to get an independent opinion on it.