As long as the lines are on the same login they will bond automatically if you send a block of IPs to both lines in the control panel.
Nice feature to have without extra charge but reading the docs on their site, it sounds like the "free" aspect of it is more like load balancing than bonding. It would be interested to see what the HTTPx1 TBB speed test shows for an AAISP "bonded" line not using tunnel end points on each side of the link between you and AAISP.
Balance yes, no problem but I dont see how you can bond tunnels over separate ISPs. Did you mean bond two AAISP lines over two different AAISP backhaul providers? i.e. One AAISP line on Talk Talk Business back haul and one on BT Wholesale back haul. That would work as tunnel would be between you and AAISP.