Concave lens is a divergent lens, so parallel light can be divergent and thickened, but it is not parallel light, and it needs convex lens to condense light, which can make divergent light parallel. You can also focus parallel light. Generally speaking, it is a condenser.

If the convex lens is in front, it will focus the parallel light. Behind the concave lens, the light will continue to diverge and cannot form parallel light.