I assume that you have added the second shift register in series with the first one. Is that the case ?
If so, then when you shift in the second set of data to the first shift register the first set of data will be shifted into the second shift register and will be displayed there. Is that what is happening ? It may help to temporarily add a delay(1000); to the start of the loop() function to help see what is happening.
If you have not added the second shift register in series with the first then how have you connected it ? If you have and the patterns do not appear as described above then check your wiring and describe what is happening.
This page has an excellent description of adding a second shift register http://arduino.cc/en/tutorial/ShiftOut and has example code to drive them.