This aseprite script non-destructively creates an outline for multiple layers as if they merged before being outlined. (It does NOT actually merge the layers). The outline is created on a new layer named "Outline"

Note: aseprite's default outline function will create an outline for each layer individually, this creates an outline for all the layers as a whole

• Generates a single outline for multiple layers without having to merge them
• Control over which cels, frames, and layers are outlined
• Customizable outline thickness and color
• Non-destructive: outline is added to a new, easily removable layer

Just download Outline Merged.lua and drop it into the scripts folder

OR

Observe there is no change to the original layers, making it easy to undo

Notice that outlines are only created in the frames with selected cells