You are completly wrong here, torrents are different and I use 2 of my lines and can get full torrent speeds using both lines combined, and its simple as routing all ports above 10000 to other isp for my torrent pc(could use lower ports and other connection balancing, but it's as simple as this) , torrents open alot of connections and balance it thru both links in this case.Is impossible to combine two ISP speed to download/upload with higer speed than the faster line.
(Unless some technology on remote side intervenes)
Is also impossible to use one line only for download and the other for upload.
The communication between same remote service can happen only on same line.
(Unless some technology on remote side intervenes)
You can only distribute the bandwidth among different devices, or different services, etc.
Example of "some technology on remote side":
Place one RouterBOARD on one datacenter and make two VPN, one using ISP1 and the other using ISP2,
and bonding together between local RouterBOARD and remote RouterBOARD.
Nat absolutely makes no difference, I use local hotspot to increase my torrent speeds, what's even funnier this hotspot blocks torrents you can't even open most of torrent sites, but I can use it just fine after I downlaod file and initialize connection thru my main link...You are not the user than open the topic.
The second line he have is already natted...
Even by doing simple load balance for each new conection evenly between 2 isps he can achieve what he wants, and torrents are best candidates for this.Oh, well... now give all the instruction and information that the OP ask, I go to bed, my help here is not needed, you are present.