My first pond had a larger peanut-shaped liner with a waterfall spout on one end. This poured into a second, smaller liner. I put my pump in the smaller pond, routed the piping up to the barrel filter, which in turn poured out into the peanut pond (so the flow of the water was filter -> larger pond -> smaller pond). There were advantages and disadvantages to this setup.
The good was that the larger fish were kept in the larger pond, and when eggs were sprayed or babies hatched out, many of them were pulled over the spout into the smaller pond, where they were able to grow without being disturbed by the larger fish.
The bad was that if sticks, leaves, or ice blocked the water spout, the top pond would overflow, causing the bottom pond to be drained by the pump. This usually happened once or twice a year, and on at least a couple occasions, I came out to find the smaller fish flopping around in an inch of water.