@huiii: This has been partially implemented already:
Issue 17 – High performance enabled SSH (HPN-SSH)

Maybe you could reuse this https://www.psc.edu/hpn-ssh-home/hpn-ssh-faq/. They say:

... is fully interoperable with other servers and clients. In addition HPN clients will be able to download faster from non HPN servers, and HPN servers will be able to receive uploads faster from non HPN clients...

@mendez: I'm not going to promise any date, sorry.

@martin: How long does stuff like this usually take to get into working version?
As in even a beta version.... Something one could download.

Sounds like a good idea....
Can't wait till it's out in production

Thanks for your post. This request has been added to the tracker:
Issue 513 – Transfer single file using multiple connections

Hi it would be great if you could incorporate a feature that could be turned on if the user needs it.
An option for multiple connections to a single file while uploading/downloading would be a great + point you could start out with maybe 4 connections.
This was we could better utilize our internet bandwidth.

Hope you'll take this into account for you're future releases.