Is there a sensible way of 'teaming' two ADSL connections?
Solution 1:
There are numerous appliances and routers (e.g. Zyxel P-663H) which will support two internet connections and balance outbound sessions across the two links.
For IP routing reasons this almost always involves NAT, although if you're already using NAT that shouldn't be an issue for you.
Balancing inbound traffic resiliently is a lot harder, since you have to publish an IP address for each inbound service, and those IPs can only ever be associated with one link.
The alternative is to swap to an ISP who can bond lines so that each line is properly bonded.