I have done a lot of reading on stereo setups, and I have never seen a standard for double-ortf. I also sometimes record in surround.
My suggestion is to get the Sabra Som stereo bar, then add two stereo bars on top of that and adjust them however you like. I got the ST-4 so I had more room. Simply take two of the adapters off. Sabra-Som ST-4 - Quad Microphone Support ST4 B&H Photo Video
Also super cheap but good is the McArthur stereo bar. MacArthur Group - Stereo Bar
Here is a photo of my setup with baby ball gags and windjammers. Those adapters on the Sabra Som will slide closer if you want.
The mics are Schoeps CMC6/MK4's.