Hello @Roman_Masson
With this approach it would be much easier to keep your eyelash mesh low poly. It will be much easier to control a small number of points without distortion, than it will be to control the same shape at high poly. For this sort of work, you will need to be proficient with the Topology Brush, ZModeler, edge creasing, and Dynamic Subdivision.
In order to do this, it would be better to draw your mesh out as a single piece to being with, rather than to try and merge those two pieces of thin, high poly geometry. That is never going to look very good. It would also in this case be easier to work with the mesh as a piece of 2d geometry without any thickness, and rely on Dynamic Subdivision to provide both a virtual smoothing preview, as well as a bit of virtual thickness to aid you while working.
So if the top and bottom part of the lashes are supposed to be one piece, draw the topology out that way. Keep the geometry minimal, and don’t add any points unless you need them to define a curve. If you want the lashes to be flat, draw them out on a flat piece of geometry then fit them to the eye. You could position a piece of planar geometry through the eye at the angle you want you want the lashes, then draw out the topology on that.
When you create your topology with the Topology Brush, reduce the Draw Size to 1 when confirming the new topology. This will create the mesh as a 2d piece of geometry without any thickness. This will make it easy to pull the points around without having to worry about an opposing surface.
When you have your piece of 2d geometry, manipulate the points with the Move and Slide tools in ZModeler. The fewer points you have, the easier it will be to shape the curves without distortion. If you positioned a plane through the eye, you can use the surface snapping features in ZModeler to snap points to that surface, keeping the lashes mostly flat. Pull the points into position to conform to the eye, and shape the curves. The topology may perform better drawn one way rather than another, so you may have to experiment.
You can use Dynamic Subdivision to preview what the mesh will look like subdivided. You can also give the mesh a “Thickness” value in the Dynamic Subdivision menu to also allow you to preview the mesh with thickness, while preserving the ability to work on it as a 2d mesh piece.
There are likely to be a number of amateur tutorials floating around the web on this process that you could find with a bit of research. There is no one right way to do it, but the tools you will need to understand are likely to be the same.
Good luck! 