I think you mis-understand.
There is not two current sources. It the keyboard is in series
with the 33's coil and power supply, all you need to do to
send to it is provide a switch break.
A small drop across the input of a optical coupler is your
receiving end on the same loop. The 33 has the supply
already built in. You should not be providing additional
power. You may have to add resistance to bring the current
down to 20ma but you are not to be providing power.
Your sending should just be a switch.
Dwight