I have three fully IP-bonded ADSL2 lines, load splitting in both directions, done by the ISP downstream and by my Firebrick router upstream. The router’s WAN (internet-facing) interface presents a single unified IPv4 address, same for IPv6. A single say TCP connection goes at triple speed in both directions, it isn't a matter of allocation flows such as TCP connections or users to one or other link, everything is really triple speed. And it’s very efficient, better than 97% iirc. I get 3.05 Mbps downstream sync rate and 512k upstream on one line, the other two are about to get upgraded modems installed and get ~2700 kbps and ~2800 kbps downstream sync rates and 436k and 496k upstream, but the downstream should increase a good bit when I upgrade the remaining two modems.
The lines are 4.55 miles long, from the best estimate I can get. BT Wholesale supplies to the ISP AA.