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 want to 2 side infuse then you will need to drill a bucket load of holes in the core to allow the resin to pass from top to bottom.

Even if I have:
bag
(spreading mat)
peel ply
fiberglass
core
firberglass
peel ply
(spreading mat)
bag,

peel plies from both sides are attached together on sides and resin will be spreaded drom side to side?
I suppose that only problem might be that one side can be faster than other?

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.

Before covering with fiberglass I will glue together two 4mm plywoods on rocker table to give shape to it, then I will make stack like I described in previous post and place it on rocker table, but just to control if it keeps shape.

I would still drill holes in the board to allow the resin to pass through, belt and braces.