They are living creatures with highly developed senses specific to their species - of course they sense each other, likely far better than we would sense one of them slithering by.

Whether they much care one way or another is a whole different question. I would assume that they instinctively categorize the other snake as "can you eat me? can I eat you? do you present any threat to me? are you a potential mate?". Other than that I doubt they do much worrying about each other.