If you answer these questions, could get a better idea of what it might be.

Are the plants both in full sun?
What has the weather been like (hot, humid, wet, dry)?
Does the water have a high ph above 7?
What is the soil like?
Did you fertilize recently?
When did the symptoms start?

Full sunlight and the weather is hot and dry ,, and i did fertilize recently ,,

Sounds like fertilizer burn. Water with good water that has a ph around 6-7. Try to wash out the excess fertilizer. Also misting the canopy won't hurt.

Yep, you have to be very careful with fertilizer on lychees. Especially when they have new growth.

They LOVE Water especially since you said it was HOT and DRY!
Water them often(if you have sandy and or porous soil) and if your soil doesn't drain quickly water a little less often but deeply...especially if they are newly planted. Good luck!

May be a good idea to setup misting emitters attached around areas of the upper trunk aimed to go into the canopy of the Lychees and Longan. The increased humidity and water, should help them out a lot in the hot and dry climate.

yes in fact i have to have a mister for every lychee or longan tree ,, as the hot and dry weather are not helping ,, i hope not to lose the trees to that ,,

Are there any mature leaves left on your trees? Lychee trees, especially newly acquired trees (recently hardened air layers) do not like a lot of fertilizer, especially non-organic N-P-K fertilizer. I lost my No Mai Tsze lychee because I fertilized it when I saw new buds of growth. I though giving it some fertilizer would help it along but all the new sprouted growths popped off and it never recovered. If you tree is more mature and there are still a good amount of leaves on it, you have a good chance for a full recovery. Good luck and please keep us updated.
Simon

Simon ,, that exactly what happened to me ,, i did fertilize wth NPK thinking that lychee and longan like micro nutrients ,, and i did fertilize a newly shipped plants which must have suffered a lot by themselves ,, and the result that i lost a hak ip lychee without knowing why at first ,, then now a sweat heart lychee and a biewkiew longan starts doing the same ,, hope they dnt die at the end ,, but lychee and longan seems to be very sensitive and jackfruit ( i lost 3 grafted trees to end up keeping them as seedlings cause the bark below graft didnt die ) ,, starfruit and sapodilla and ice cream bean are nice with as much fertilizer as possible ,,