Is it possible to program a few Arduinos at the same time?
Arduino Pro Mini is programmed through serial, using a bootloader. It uses STK500 protocol, which is described by Atmel. 

In theory, it can be possible to program multiple boards if only one TX Arduino pin is connected to USB-Serial converter, and all RX are in parralel, but this is an unreliable method. Only the board with TX pin connected can send its status back to programmer.

Arduino Mini also requires a physical reset before uploading. Make sure that all Reset pins are connected together when reset button is pressed.