You can use 2 network adaptors with Linux for using 2 ADSL connections. The first one, you can connect and use with the host operating system. After this is established and working, start your VirtualBox guest OS and choose the network adaptor as the other ADSL connection. It could be a different ISP. This should be in Bridged mode and use the adaptor that is not in use on the host operating system.
You will now see traffic going through 2 gateways. Note that it is not possible for you to join both gateways to increase your bandwidth because at one time, there can be only one default gateway.