How are "parallel fields" implemented on the Hubble Space Telescope?
As the question Instrument aperture sizes on Hubble Telescope shows, the focal plane area is large enough to focus on several instruments at the same time (but with each capturing a different area).
If two objects of interest are separated by a certain angle (the instruments are fixed within the focal plane), the telescope can be rotated so that two instruments are exposed, one to each object.  If desired, the telescope can later be rotated to swap the instruments on each object, so that both objects get a wider spectral coverage, for example.  

No optical components point the instruments (cameras/spectrometers) at different objects; rather, the whole telescope orients on one direction in space, and by rotating around the main mirror axis to a particular angle (matching the positions of the fixed instruments) can choose to expose multiple instruments to multiple objects in that Field-Of-View.  All instruments (except the WFC UV/IR sensors) are always exposed to something, but data collection/transmission stresses the systems, so 'parallel' viewing is used selectively.  

Only the Wide Field Camera uses a pick-off mirror (steerable for UV or IR sensors). The other instruments are aligned with their patches of the focal plane and used COSTAR lenses for correction until Service Mission 4 removed COSTAR and replaced the actual instruments (which now have built in corrections). The Wide Field Camera was replaced several times and had its own corrective mirrors instead of a lens via COSTAR.