how do you connect the second mouse? via some com port? because if you connect both via usb and have one "cursor" you can't. only with com port+usb/ps2 port you will be able to control 2 separate mouse cursors at the same time
check
this thread out, bugs_bugger explains this thing
but i think that the game supported joystick for 2nd player, you can map your joystick movements to keys in dosbox (ctrl+f1) and play via keyboard which the game will think that is joystick