dual monitor display through vga with macbook pro in clamshell mode

I have my macbook pro (early 2011) in clamshell mode hooked up to a dual display 2 port USB KVMP switch. The connection chain is: macbook pro - mini to vga adapter - vga splitter - IOGEAR miniview KVMP switch - dual monitors. When I switch the port to the windows computer both monitors work together as one large display. When switched to mac both screens are the same. How can I get both screens to work as one display when switched to the mac?