Bonding is only for aggregating a VPN-Tunnel … you can become 40 MBit Internet only if you are able to connect your Bond to another Side, that has 40 Mbit (or more) Bandwidth to the Internet.
And if you are Using ZeroShell you MUST have 2 Lines with the same speed (round robin) or you will end up with the speed of the slowest Line ( * 2 ).
If you have no access to a 40 MBit Side (e.g. a Datacenter), you should use the ZeroShells “Net Balancer”. This will NOT increase the Bandwidth for a single connection, but balances many connections over your 2 Lines.