Getting different ISP's lines installed and then bonding on the top sounds good in theory, but it does mean you completely lose control of the entire connection.
Trying to diagnose where a bonded ADSL problem is and whether its with the internet transit or core bonding hardware (which has to go through one supplier) or the box itself (which you may or may not own or have configured) or one or more of the individual lines is a nightmare.
It's far better to have everything end to end with one supplier to reduce the latency and jitter in general, and also mean the buck stops with that bonded ADSL supplier.
You can bond over the internet, but the path the data takes is far from in anyone's control, so it just makes more sense to do it within an ISP's network.
While you're at it, go for one that doesn't contend their lines! Why take a line that slows down and combine it with another that slows down. Contention and bonding don't mix!