Done well, layering adds depth and lets a small special piece live in a big room. Done badly it looks like you could not decide. The difference comes down to three decisions, and none of them is about pattern.
Contrast the scale, not the colour
The base layer should be large, quiet and textural — a flatweave, a jute, a plain wool. The top layer is the piece you actually want people to look at. If both layers are patterned they compete, and the eye reads the result as clutter rather than as composition.
“The base layer is there to be quiet. The top layer is the one you want people to look at.”
Leave a deliberate margin
Show at least twelve to eighteen inches of the base layer on every side of the top rug. Anything less looks like a mistake in placement rather than a choice. Keep the margin even. An off-centre top layer reads as accidental unless the asymmetry is obviously intentional and repeated elsewhere in the room.
Solve the trip hazard first
A top layer with a thick pile sitting on a flatweave will lift at the corners. Use a thin pad between the layers, not just underneath the bottom one. If the top piece is antique or fragile, this is also what keeps the two textiles from abrading each other every time somebody walks across them.
