VirtualBox: two network interfaces (NAT and host-only ones) in a Debian guest on Ubuntu

I created a Debian VM on VirtualBox with two interfaces: a NAT one (for accessing internet) and a host-only one. However, I do not know how to make both interfaces work at the same time. If I the define the host-only as the adapter 1, I can access my VM from the host but not the internet; if I define the NAT one as adapter 1, I can access the internet but cannot reach my guest Debian.

Set Ubuntu System Proxy Settings without Restart from commandline

I am using Ubuntu 14.04 . I want to change the http proxy settings from the command line. This should be equivalent to changing in the GUI(All Settings->Network->Network Proxy) and clicking the button Apply System Wide. I don’t want to restart/logout the system as I am planning to change the settings dynamically from a script(bash).