Okay, here's what I found:

"What can be Grafted?

Most varieties of a particular fruit species are interchangeable and can be grafted. Because of differences in vigor, some are better able to support others as understocks. Plants of the same genus and species can usually be grafted even though they are a different variety. Plants with the same genus but of a different species may often be grafted. But the result may be weak, short-lived, or they may not unite at all. Plants of different genera are less successfully grafted although there are some cases where this is possible. Plants of different families cannot be grafted successfully."

"Reasons for Graft Failure

1. Stock and scion were not compatible.
2. The cambiums were not meeting properly.
3. Scions were upside down.
4. Grafting was done at the wrong time.
5. Understock or scion were not healthy.
6. Scions were dried out or injured by cold.
7. Scions were not dormant.
8. The graft was not properly covered.
9. The scion was displaced by storm, birds, or other means.
10. The graft was shaded too much by other growth.
11. The graft was attacked by insects or disease.
12. The graft union was girdled because tape was not cut or released in time."

Longans are in the same family as lychees, so you may have a bit of success...maybe. Keep in mind that lychees are notoriously difficult to graft within the same cultivar, yet alone outside the genus. It's certainly worth a shot, though. You may just be better off growing two separate, dwarf fruit trees. Different longans, however, should all be compatible with one another, and the same goes for lychees.

im not sure how a single tree would react to having two stems blooming at different times, lychee/longan are already unpredictable bloomers.

let me know how it goes i always wondered if i could graft different lychee varieties onto my sweetheart.