The Peplink Balance One does not support SpeedFusion Bonding out of the box.
SpeedFusion Bonding also requires you to build a VPN tunnel to a second Peplink end-point with an internet connection that is atleast as fast as the speed you’re trying to bond, in this case 6 Mbps.
This article might help understanding the SpeedFusion Bonding technology:
It might be that you’ve seen a 6 Mbps Speedtest.net result after adding your second DSL line.
If that’s the case, please read this post about why Speedtest.net is basically fooling you:
The Peplink Balance One does support loadbalancing out of the box however, so you could make an outbound loadbalancing rule with the Weighted Balance algorithm.
This way Peplink Balance will evenly distribute all traffic across your available WAN connections.
This does not bond the WAN connections, but rather uses them side by side.