Welll im not sure sure how to answer the 1st part of your question but I can have a go at the 2nd.

There are a couple of different ways depending on what you use (NURBS or Polys).

In Polys you can model the first half then duplicate it and combine the two pieces, then stich the seam.

In NURBS it is a little more difficult.

If you want to have two pieces and have the changes that you make to one apear on the other side in real time you should do the following.

Select the 1st half, goto Edit --> (Duplicate Option Box) and make the 'Instance' box checked. Then duplicate. Then any changes you make to the 1st half will be mirrored on the 2nd.

I hope this helps.