Spotlight

Multi View

One frame split two, three or four ways, a different source in each. A diptych side by side or stacked, a triptych either way, or a quadrant.

One of the tools in Aspects for macOS. $19 once, cloud optional.

Three layouts
Diptych, triptych or quadrant, with a horizontal or vertical axis for the first two. Five arrangements from two controls.
Equal splits, on purpose
It is a tool for putting a few pictures into one frame, not a canvas for arranging them. Panes divide the frame evenly and boundaries are computed so no seam falls between them.
Each pane fits itself
Every pane keeps its own fit, scale and position, so a portrait and a landscape can sit together without either being squashed.
The gap is the background
Space between panes shows the artboard's own background rather than a colour of its own, so there is no second colour to keep in step.
Effects per pane and per frame
Brightness, contrast, saturation and grain apply to each source before it is placed, to the finished composite, or to both.
The frame you already have
Margins, background, layout grid, transparency and export formats are the same ones every other tool uses, because the split is all this adds.