Controlling electric power steering using an Arduino to emulate torque sensor
Looks like you are not driving the output drivers are not strong enough. Many of the sensors have low output impedance (about a kiloohm or less) and thus can provide tens of mA's of current. Your 100K digipot can provide much less than that. 

Try adding op-amps to each output in a simple buffer connection. Make sure the op-amp can have the desired input and output voltage (you likely want 'rail-to-rail' opamp) -- for example LM324 will only accept 0V to 3.5V input when powered from +5V.