@Saffron: If you look at the planned features list in the original post you can see that overlapping grids is a planned feature, but it has not been implemented yet. It is not something that is trivial to add, and will mean having to modify features, such as pathfinding and visibility tracing quite heavily. Depending on how proficient you are with blueprints I must warn you that adding it yourself might be a pretty big challenge. If you need this feature now and cannot wait a month or two for me to implement it while you work on other parts of the game I wish you luck, and will be very interested in following your progress.
If you’re interested, this is the way I am currently planning to implement multilayered grids, though this is just a rough sketch that I haven’t tested the viability of yet: I would create a new sort of actor called a subgrid that would share some, but not all features of the Grid Manager. It would have its own arrays for things like walkability and the position of pawns as well as its own array that would store what indexes in the subgrid correspond to what indexes in the Grid Manager in their XY location (this would be calculated in a similar way to how tiles and units placed on the grid are assigned their indexes). Pathfinding, visibility etc. would be driven from functions in the Grid Manager. Connection tiles would have to be designated for where the Grid Manager is connected to a subgrid, either in its own array or as a separate up/down direction in the Edge Costs array that would be checked during pathfinding. For visibility the same indexes would be checked on both the Grid Manager and the subgrid, using a macro to translate indexes on the Grid Manager to the corresponding indexes on the subgrid.
That’s roughly what I’ve got planned at the moment, but I’ve got a few more features to add before I start working on it myself (check the planned features list). I might end up doing something else entirely, but I can’t be sure until I start experimenting.