It will work and it will work with single sided squares. Two factors may be affecting your renders. First, no "simi-shadows" from gray areas of shadow map - they are either on or off - only transparency map luminance values above 40% will cast shadows and all cast shadows are opaque. You can however, soften the shadow edges and blend them together (3rd image) by using a larger view from the light's ShadowCam. This may also be your second problem. If the area for the cast shadows is too small in the view from the light's ShadowCam, then there will be too few pixels covered and therefore a low-res shadow that is aliased too much and too soft to be visible. If so, move the light closer or zoom & reposition the light's ShadowCam.