func main() {
    c := make(chan []int)
    for i :=0; i < *split; i++ {
        // call goroutine
        go twice(list, prefix, i, c);
    }

    for i :=0; i < *split; i++ {
        // join twice routins
        <- c
    }
}

func twice(list []int, prefix int, index int, c chan []int) {
    for i := 0; i < prefix; i++ {
        list[prefix*index+i] = list[prefix*index+i] * 2;
    }
    c <- list
}