Help if you can to set up 2 monitors (tower won't detect or identify 2 monitors)

I have Dell Optiplex tower and two Sansung monitors. Both monitors work well individually using the HDMI ports. Also%2C using a spliter%2C both monitors work duplicated. I need the expanded workspace. Ports are HDMI and VGA. I also bought a HDMI to VGA connector and it does not allow detection of two monitors when I use both the VGA to one monitor and the HDMI to the other monitor. What gives%3F Please HELP. Do I need so something else%3F or just help connecting the cables correctly