I would strongly advise that you model your DC motor in Simscape as well rather than Simulink, this will avoid algebraic loops and the interface problem you are asking about.
However, if you really want to do this, I would suggest using the torque output of the motor model, feed it to an Ideal Torque Source block via a Simulink-PS Converter block. I would then measure the rotational speed on the mechanical side using an Ideal Rotational Motion Sensor block and feed it as an input to the motor model via a PS-Simulink Converter block. You can't specify both the speed and torque of the motor at the same time, if one's an output, the other one has to be an input.
HTH,
Arnaud
