I am going to make kiteboard.
As core I have 8mm plywood and I want to cover it with fiberglass.

Question is if it may be possible to make it in one shot. I found video where is shown infusion of kiteboard but it is not told if it have been done with one shot.
https://www.youtube.com/watch?v=7NFdIi9CJ_M
With such small distances do I still need resin spreading mat?

Another question is if it is better to apply one thick layer of fiberglass or two thinner layers?

If you have infusion mesh top and bottom then it should be OK but you will need to take care with inlet and outlet positions.