I'm confused...
That is: three places to put a wrench, which two do I use?
You can see a seam where the large hex part (guessing it would be a 7/8") meets the section that has flats for a wrench on it. There is probably a check valve or something in there. You could probably take it apart without hurting anything, but it shouldn't be necessary. I always put the smaller fitting in the vice because it's more apt to round off with a wrench and a vice can apply a lot more pressure without distorting it.
Super, thanks to all. I did get it apart though I was still rounding the brass fitting despite the vise!