jIan,
The Toto Aquia use a split button for the flush.
I have sold 150 of these over the last three years.
They have been rock solid. One of the most trouble free toilets on the market.
I sell thousands of toilets every year. The Toto Aquia is my favorite dual flush toilet out there.
The twoflush should work for most standard toilets to reduce water usage.
If the toilet was an average toilet before, it's not going to improve anything. It will just flush with less water, and with less power.
The advantage of a toilet designed to work as a dual, is that the bowl is structured to work with less. I had the Toto Aquia installed in the main bath for two years, and it never plugged.
It would flush just about anything even on the 0.9 flush.
Some of the other methods for reducing water usage in old bowls, it to put a glass jar in the tank to reduce volumne.
Or to use a quicker closing flapper.
There are the dual flush flush valves on the market for old stuff too.
However, if the old bowl was rated at 200 grams or so the old way, reducing water will also reduce that number.