For a pond around 1000 gallons, I’d personally lean toward two 600 gph pumps instead of one 1000 gph pump.
The main reason is redundancy. If one pump fails, the other one is still moving water and providing some circulation and oxygen. With a single pump, a failure means everything stops until you notice and replace it. In outdoor ponds that can matter more than people think, especially in warm weather.
Running two pumps also gives you a bit more flexibility. You can split the flow between a filter and a small waterfall or fountain, or place them on opposite sides of the pond to avoid dead spots where debris settles.
The only trade-off is slightly more maintenance and power use, but for a pond your size it’s usually minor. Just make sure the combined flow rate isn’t too aggressive for your filtration setup.
One small tip: whatever route you go, try to pick a reliable model designed for continuous pond use. I’ve had good luck with some of the Best Pond Pumps From EasyPro because they tend to run pretty quietly and hold up well in outdoor setups.
Takeaway: two pumps = safer and more flexible, especially for a DIY block-and-liner pond like the one you’re building.