You either need a USB to VGA adapter or Matrox's DualHead2Go. Be warned that USB adapters are slow so fine for Word or email, but not good YouTube, movies, Netflix, etc. And DualHead2Go is not cheap. (Around $150.)
You didn't mention the specifics of your monitor, but have you checked if your current monitor(s) have both DVI and VGA? Many do. If at least one of them has DVI, then assuming the Mini you're "about the purchase" is a new 2011 Mini, you can get a HDMI to DVI adapter. Current Minis have HDMI and Displayport. Then you can plug the DVI monitor into the HDMI port and the other monitor into the Displayport. (The Displayport will work with DVI or VGA with the appropriate adapter.)