You are correct; to create a hybrid there must be sexual reproduction either natural or transgenic. However, I'm suggesting that it’s possible for two corals to fuse together for reciprocal-altruistic or mutualistic reasons, a "you scratch my back and I'll scratch yours" type of relationship.
The genetic structure of both corals stays the same; we still have two unaltered genotypes, however, because they are so closely related they can cohabitate on the same skeleton and share resources like nutrients. The adaptive advantage would be the ability to handle two different environment regimes.
Purely hypothetical example:
Monti "A" is best adapted for lower flow; its polyp structure is such that it traps food particles best at water velocities lower than 0.5 Meter/Second. Monti "B" is the opposite and is at its optimum productivity and growth at velocities greater than 2.0 M/S.
Alone on the reef, the the two corals grow individually. In the summer the ocean currents average 0.3 m/s and "A" thrives growing quickly and unable to even utilize all the nutrients it gathers. However, "B" is stunted and in danger of being over taken by other species of corals.
In the winter, the roles reverse, currents pick up to average over 3.5 M/S and "B" takes off while "A"s polyps can't handle the extreme currents and begins to die back.
However, one day a piece of "A" falls on top of "B" and the two corals begin to fuse. As the two different polyps begin to replicate they intermingle with one another so that an "A" polyp is always next to a "B" polyp. They're closely related enough to share nutrients through normal transport mechanisms, intercellular or otherwise, yet are still seperate organisms, perhaps even different species. In the summer "A" shares with "B" and in the winter "B" shares with "A". Both corals sacrifice some individual growth for part of the year but gain the adaptive advantage of more consistent growth year round preventing it from being overgrown by the competition.
Of course this is all theory, I've never read any papers about two corals utilizing mutualism, co-operation, or reciprocal altruism but it’s possible. There are countless examples of this type of relationship in nature. Here is a paper on reef sponges doing pretty the same thing I just explained.
http://links.jstor.org/sici?sici=0012-9658(199701)78%3A1%3C146%3AMASOCR%3E2.0.CO%3B2-I
