The shift registers are normally connected after each other so you just continue to shift out more bits before you strobe the scanline and tell the shift register to start emitting the values on the parallell pins. There should be no significant difference to display 80 diodes in sequence than just 8 diodes. It's just a question about making sure that you are fast enough to refresh at required frequency (if the text are flowing) and to possibly control any output-enable so that you can modify the display intensity depending on external light.