Initially, you want to segregate any new additions; keep any new herps away from established ones for at least one month in another section of the house if it's feasible. This is to make sure the new addition is not likely to pass anything (Bugs, disease...) to the rest of your collection.
Snakes are intrinsically solitary; they don't enjoy company. Most of the time they will tolerate other snakes in the same enclosure just fine, but if it is at all possible it is best to keep them separated unless you want them together for a specific reason, such as breeding. Adding a snake to an already established habitat will cause some stress; like I said, most of the time it's minimal and both snakes will tolerate just fine, but sometimes the stress will be to much causing illness, feeding problems.... etc..
Some other disadvantages of keeping them together is you will not be able to track which one is deficating, if one gets sick they both could, and bugs (mites...) can spread easily to both.
Feeding is another thing. When you feed both, and place them back in the enclosure together they both still have food on the mind, and smell in the air and on both snakes. I have personally had a
BCI strike/constrict another
BCI directly after feeding because of this.
I'm not saying it's a terrible thing to put two together (I have four or five enclosures with a couple inhabitants myself) there are just a few things to consider. If it is at all possible, keep them separate is my advice.
