Stacking two animations in a page always drifts apart, because each keeps its own clock. This decodes both, merges their frame boundaries into a single timeline, and re-encodes one file — so they can't drift, ever.
The first is the base and sets the canvas; the second is laid over it. Click a slot to browse instead.
Arrow keys move the selected layer by one pixel, with Shift for ten. Use the Move and size controls to choose which layer.
Drag on the preview, or focus it and use the arrow keys — Shift for ten pixels. Ctrl+Z or ⌘Z undoes a move.