Since SimMechanics 2G outputs physical signals, all you need is 2 Simscape blocks to make this type of connection: Ideal Translational/Rotational Motion Sensor and Ideal Torque/Force Source.
In other words, you measure the speed of a SimMechanics joint, apply it to Simscape, measure the force/torque on the Simscape side, and apply it to SimMechanics.
... Easiest thing to do it to download the library at this File Exchange link which contains pre-made versions of these interface blocks. - Sebastian
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