No, you do not need two waste ink tanks for the 7800/9800 or 7880/9880 like you do with the 3800/3880.
To reuse the waste ink tank for a 7800/9800 or 7880/9880:
1. Pry the plastic grid off the top (using a straight edge screw driver or something similar- it's held in place with snap in tabs), remove + discard the ink soaked pads.
2. Rinse out the empty tank with warm water and dry with paper towels (get the outside, especially the chip area, nice and dry before reinstalling into the printer, the inside doesn't matter).
3. Take three paper towels and fold them together lengthwise in thirds (so you have a pad of paper towels about the same size and shape as the tank).
NOTE: I use paper towels, but have heard of people using a diaper or sanitary napkin as absorbent material when reusing the waste ink tank- it doesn't need to absorb all the waste ink, but you want something in there
to stop the ink from splashing around.
4. Loosely place the folded paper towels into the empty tank, then snap the plastic grid back on the top (the thick end that says "EPSON" goes on the end of the tank that is facing the outside of the printer).
5. Reset the waste tank chip using the chip resetter.*The "LF" chip resetter will reset the large format refillable cartridge chips to read full and waste tank chip to read empty.
Best regards and happy printing~ Dana