In the MH53E world, the right seat is the pilot seat, and the left seat is the copilot seat. Most of the time in the Navy 53, the HAC sits left and copilot sits right. When I went through HMT-302 (with the USMC CH bubbas), it was the opposite, so I guess in the long run it doesn't matter. Some switches and whatnot (APU) on the right side only.
It is a good questions why 2 pilots are required. There are some mission doing vertrep (which we don't do in 53s) where a pilot swap is necessary, but most of the time the other pilot just functions as a copilot (swithcing radios, yadda yadda yadda). Gatordev and HH-60H have good points about why.