How much flow can a 2" pipe support?
 

Michael Shaffer writes:

I buried a 2" pipe between my veggie pond and main pond. I think the
Pondmaster 3600gph pump might come down in price now that the newer
model is out, so I was thinking of getting a second one. Do you think my
2" pipe would support 7200gph?

Depends how long, how many fittings, static head and what flow is acceptable...
http://www.mdminc.com/friction_loss_chart.htm

You say one pipe. How does the water go the other way -- stream?