With the first layer of grass fully dry the longer grass can be added using a layering spray adhesive. I've used Peco's offering but others are available. The grass fibres used are again from Greenscene and are medium length 'Straw'. The areas where the longer grass is wanted are sprayed with the layering spray. care needs to be taken as the spray does exactly that and will go everywhere, coating areas in adhesive where you may not want grass fibres to stick. The solution is to use a mask(s) of paper to prevent the overspray getting to where you don't want it to. Once the layering spray has been applied the static grass applicator can be loaded with the longer straw fibres and be applied liberally over those areas. It ends up looking like this.
Not very realistic as it stands, or indeed doesn't stand. Many of the fibres are actually laying on top of others that are upright and have stuck to the layering spray adhesive. Much of the excess can be removed in this case by holding the diorama upside down and tapping or by giving it a gentle vacuum.