If you layer two batches of potting mix in one container, and they are rather different in grain size or peat content, scratch an inch or two of the new mix into the top 2 inches of the old mix, so the transition between the two mixes is gradual.
That will help water migrate easily, and divide itself evenly between the top and bottom of the pot. Abrupt changes between two very different mixes can interfere with capillary movement.
