Support has recently landed in Mir master for logical output groups. When multiple outputs are part of the same group, they are treated as a single logical display. Maximized, attached and fullscreen windows are stretched across the bounds of all of the outputs in a group.
Output groups can be configured in any Mir compositor that uses a *.display YAML configuration file. Add
group: 1 to all outputs you want to be part of the group (or use multiple non-zero values for multiple output groups). Outputs that are part of a group are configured and positioned normally.
Below is an example of two outputs in a group: