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I do not know of a way to distinguish by which node sent a message in the callback on the subscriber side.

I suggest you make your two publishing nodes publish to two different topics and your subscriber then subscribe to both indivially. Remapping topics is your friend here.

I do not know of a way to distinguish by which node sent a message in the callback on the subscriber side.

I suggest you make your two publishing nodes publish to two different topics and your subscriber then subscribe to both indivially. Remapping topics is your friend here.

Edit:

In response to your syntax problem: Have you read the actual error message? It probably indicates that you forgot to close the <remap> tag. You can put it into a group, but I believe it is clearer to put it directly inside the node tag. Also inside a rules file I would not use the remapping syntax with command line arguments like you do for scan. This is just confusing. Try this:

<launch>
  <node pkg="leg_detector" type="leg_detector" name="leg_detector_front" args="$(find leg_detector)/config/trained_leg_detector.yaml" output="screen">
    <remap from="scan" to="scan_front" />
    <remap from="leg_tracker_measurements" to="leg_tracker_measurements_front" />
  </node>

  <node pkg="leg_detector" type="leg_detector" name="leg_detector_rear" args="$(find leg_detector)/config/trained_leg_detector.yaml" output="screen">
    <remap from="scan" to="scan_rear" />
    <remap from="leg_tracker_measurements" to="leg_tracker_measurements_rear" />
  </node>
</launch>

Or alternatively and maybe even better in this case with namespaces, like suggested in another answer below:

<launch>
  <node pkg="leg_detector" type="leg_detector" name="leg_detector_front" args="$(find leg_detector)/config/trained_leg_detector.yaml" output="screen" ns="front" />
  <node pkg="leg_detector" type="leg_detector" name="leg_detector_rear" args="$(find leg_detector)/config/trained_leg_detector.yaml" output="screen" ns="rear" />
</launch>