Betta splendens do occupy the same body of water in the wild, if not they would not breed. If you had a large enough tank set up in a similar manner to their natural environment there is no reason you could not keep them together. The question is how large of a tank would be needed?
I've asked this before, and have yet to get any sort of answer, you would think the more serious B. splendens keepers here would have the curiosity as well as experience needed to see just how much territory these fish do require. This would also aid in determining a suitable size tank for a single species, as well as certain particulars about setups that would be more compatible to this species. As with any species there will be variables where aggression is concerned, with other species this has been worked on for many years, for whatever reason B. splendens has fallen to the wayside.
B. splendens are not a hybrid in the terminology normally associated with fish, nor are ornamental bettas with the flowing fins the sort that are bred for fighting. A hybrid is most often associated with crossing species from a different taxa, such as crossing B. splendens with B. pugnax. This is an often debated area, as some purists with certain species will do their best to keep members of that species from a certain location when breeding is concerned. They will be from the same taxa, but have traits particular to that location that breeders try to keep intact. Just the same as ornamental bettas are bred mainly for appearance, bettas bred for fighting are oriented more towards strength, endurance, and aggression.
Bottom line; you can keep them in the same tank, how large of a tank you would need has yet to be worked out. Without considerable experience it is not someting to try.