Is there any risk if we stand in between two large conducting plates (capacitor) connected to supply?
The first danger to consider is the danger of dielectric breakdown, and consequent current through the body between the plates.
If however, that danger were somehow eliminated, the next danger I would consider applies to an alternating electric field. Depending upon frequency and amplitude, a body between two conductive plates with an alternative electric field could cause heating in the body, or unwanted electrical activity in the nervous system.
Heating would not be a problem with a static (non-alternating) electric field. However, effects upon the nervous system might still be present. This is an area of current research (see for example this article). Or just google &quot;nervous activity in high voltage fields&quot; or something similar. I believe that at present, our knowledge of the effects of strong electric fields upon the nervous system is rather sparse.