This was really just a happy accident. I did this when I was new back into reefkeeping last year. I bought a frag pack and it included pink and green ORA Birdsnest, and the growth tips broke off in shipping, and I was too cheap to throw them away! I just glued them next to each other and the pic is what resulted.
I am not an expert period, but my guess is this: human stem cells are young and undifferentiated, meaning they have the potential to become any type of specialized cell in our bodies and in some cases be transplanted from one person to another, but after they change they will reject any cells outside their own body. I am wondering if growth tips of corals may be similar to stem cells, particularly ones that are in the same genus but different species, like pink and green birdsnest, and that the growth tips may not be differentiated enough to reject or kill each other, the way mature but dissimilar acopora species do.
I don't know you could graft, say the growth tips of a milli and tort, but maybe you could the growth tips of two different millis.
Dave