I acomplished the same thing by mounting a few fishing weights
on the Flapper valve. Actually I mounted them to the flapper
lift chain, low, down near the flapper so that when the flush handle
is released, the weights press on the flapper.
Anyway, a quick push and release of the flush handle causes the
valve to open and close. That only releases enough water for
a #1 flush. Holding the handle down gives a good long flush
for #2.