The lftp program can handle both multiple network protocols and simultaneous connections.
Lftp supports the following protocols:
- http and https
Further information on ftps etc here.
To download a single large file located at
<URI> using 2 simultaneous connections:
lftp -c "pget -c -n 2 <URI>"
-c option specifies the command(s) to run.
pget establishes a file transfer using several parallel connections. The second
-c option means continue transfer if this transfer had previously been interrupted. The
-n option limits the maximum number of connections to use.
There is more information on commands and options in the online man page.
Warning: This can speedup transfers, but loads the network and servers heavily. Speedups are contingent on network and server conditions.
To address the practical example in the question:
lftp -c "pget -c -n 2 sftp://email@example.com/~/backups/large.tar.gz"
This example downloads a large backup file over ssh (sftp) with 2 simultaneous connections. It assumes you have password-less ssh login setup.
Note: Lftp can also be used in an interactive mode; similar to other ftp commands.