TWO Balls in Play

[JPaco54]

Last night - St Louis Cardinal game, pass ball on ball 4, BR sprints to 1B, F2runs down the ball to the backstop...F2 turns and throws to 1B to catch runner rounding 1B, runner safe sliding back into 1b, meanwhile, while F2 is throwing to 1B, PU tosses another ball to F1...which ball is in play?...what if ball was thrown to RF....which ball is in play...the announcers stated the ball the PU put into play because PU would not want to be the cause of the error. This gets back to the issue as to who or whom should throw the ball back to the pitcher. Just something that I had not seen before.