2 SW_Serial problem

Using Serial and one softwareserial works perfectly well so far. I only run into troubles when I use 2 softwareSerials at the same time. With your AltSoftSerial I'd also read from Serial and write on SoftwareSerial?

Here a sketch that works perfectly well but uses the hardware UART-pins so when the USB-Mini is connected, then it reads/writes there...

#include <SoftwareSerial.h>

//Software-UART Pins for XBee SX 868
SoftwareSerial xbee_serial(4, 5); // RX, TX

void setup() {

  Serial.begin(9600);
  while (!Serial) {
    ;
  }
  xbee_serial.begin(9600);
}

void loop() {

  //Input, which comes via USB-Mini is transfered to SWSerial
  if (Serial.available()) {
    xbee_serial.write(Serial.read());
  }

  //Input, which comes via SWSerial is transfered to USB-Mini
  if (xbee_serial.available()) {
    Serial.write(xbee_serial.read());
  }
}