Not sure if I follow your question 1. In dual mac mode you will need a cable for each independent interface. In switch mode, only one cable should be used.
2. The command is ifup, for example to bring up eth1 you would type "ifup eth1", this will start a sequence that enables the port, tells the phy to connect and to start a dhcp process. Before dual emac works though your question 3 is required. The example link you are pointing to was intended to demonstrate two different ways to use the second port on the evm-sk board that TI provides.
3. to config configure the kernel for dual mac use depends on which SDK you are using.
For SDK 6.0, a kernel configuration is required
http://processors.wiki.ti.com/index.php/AM335x_CPSW_(Ethernet)_Driver's_Guide#Compiling_kernel
For SDK 7.0 a device tree entry is required, not
http://processors.wiki.ti.com/index.php/Linux_Core_CPSW_User's_Guide#Dual_EMAC_Device_tree_entry