If price is a consideration, you might consider two 10 TB hard drives on machines on two home gbps Internet connections. It's highly unlikely that both would go down at the same time, unless they were in the same area, on the same ISP.
Just use two A records for the one DNS name, and let the clients choose.
The other way is to have two names, like dl1 and dl2, and have your download web page offer alternating links, depending on how the downloads are handled.
You very rarely can do multi-ISP bonding, often not even with multiple lines from the same ISP, unfortunately.
