When using the ROM routines you need to open the channel to display things on the bottom two lines. Putting 0 or 1 into the accumulator and calling 5633 will do the job. When you want to display on the rest of the screen, call 5633 with 2 in the accumulator.
Once you've opened the lower screen for printing, make sure you reset the cursor position to the top left. Printing chr$ 22, chr$ 0 and then chr$ 0 will do that. Then you should be able to put another 64 characters in the bottom panel. Don't go beyond that or that part of the screen will scroll up