Same simple thing I tell everyone who want to mix species. One thing which is a sure thing is that the bacterial content in the gut of different species vary in levels. When one snake poops it releases some of this bacteria into the enclosure which the other snake may get contaminated with. Now if the other snake somehow ingests some of this bacteria (via water bowl, digging through substrate) it upsets that snake's intestinal bacterial fauna and you may have a sick snake on your hands.
I agree with Chuck (finally! lol) as rattlers have been found to be pretty gregarious. Let's even discuss den sites but mom's have shown maternal instincts by keeping her babies in a rookerie and preventing them from wandering off by budging them back in. Sometimes there are communal rookeries with a few post-parturition moms sharing one spot for this purpose alone.
The Herp Room
"The day I tried to live, I wallowed in the blood and mud with all the other pigs" - C. Cornell