current-sourceoperational-amplifier
simulate this circuit – Schematic created using CircuitLab
The above circuit delivers a predetermined current to the load:
$$i_L=-\frac{u_s}{2R}$$
Is it possible to extend it, so that it does the same thing, but for two diffrent loads, while using only one opamp (and resistors)?
Basically, I am looking for a way to create two independent current sources.
You are making your life unnecessarily difficult. Instead of switching both sense resistors,
simulate this circuit – Schematic created using CircuitLab
Also, you should look carefully at your MOSFET's Rds(on) rating. Usually, the rating is given at large currents. Since you are only dealing with 100 mA max, you can probably get away with a cheaper FET.
Yes, you can parallel current sources to get a combined current source that is the sum of the two. This is analogous to putting voltage sources in series to get a combined source that is the sum of the two voltages.
There is no issue of balancing the current sources. Each will sink the current it is set to sink. There is nothing wrong, for example, with combining 5 mA and 20 mA current sources to make a 25 mA current source.
One issue to watch out for is the compliance range of each current source. That is the voltage over which a current source still works like a current source. The voltage the two current sources settle at must be within the compliance range of both sources. If it's not, then at least one of them isn't a current source anymore.
Best Answer
As Andy states you will need an op-amp per current source. One can simply not direct a specific current in more than one direction.