I have a pond that is 300' x80' x20' deep I am interested in how far apart I should set my two bottom diffuser for optimal performance.

That depends on the underwater topography and any submerged structure.

No submerged structure. Pond slopes pretty steeply then flattens out. There will probably be some aquatic vegetation this year but mainly around the edges.

I also put a 45 degree bend on the jet of my jet ski and ride around the pond for about 20 minutes once or twice a week. That pushes the surface water down to the bottom pretty well.

If the pond is essentially oval shaped, and has smooth contours underwater, I'd put the diffusers at the edges of the deepest water....Alternatively, I would put one diffuser in the deepest spot, and the second near the pond's middle point, as long as the water depth is at least 10 feet deep.

In either case, use valves so you can adjust air flow to each diffuser....one diffuser will always try becoming an air hog...

Thanks rainman the pond is just about a rectangle. The east end is shallower. So I thought I'd put the first one about 60' from the west and the other about 80' from it that puts both in about 20' of water.

Rainman has good advice on this. The two diffusers should be spaced apart in the deepest part of the basin. IMO at 20 ft deep each diffuser will have a circular influence of around 80-100ft.

Thanks that is what I was hoping for. That would cover most of the water that really needs to be treated. Between the aeration and my jet ski aeration I think I should have pretty good coverage.