Basic concepts:
Google Voice is a service, just like Gmail, Docs, Calendar, Maps, etc. All Google services are accessed via a single Google account user ID and password.
A Google Voice account can have one
inbound telephone number, or two inbound telephone numbers. The inbound telephone number is the number people call to reach you.
If you start out with a freebie number from Google's pool, and then you port in a number, you now have two inbound Google Voice numbers. The ported-in number becomes the "Primary" number. The original (freebie) inbound number goes into a temporary, "death row" status for 90 days. During that time, you can either tell your contacts to start calling you on your ported-in number, or you can pay a fee to keep the original number, too. After paying the "keep the number" fee, it becomes a permanent, secondary number.
If you start out with a freebie number, port in one number, and then want to port in another number, then yes, you will lose the freebie number, since an account can have a maximum of two inbound numbers.
If you port in a second Google Voice number to the same account, then all inbound calls made to either GV number will forward to the same destinations as the first number, and callers will hear the same voicemail greeting. There is no way to handle the inbound calls differently, depending on which number is called.
One number is designated as "Primary", and the other "Secondary". You can swap primary vs. secondary roles at any time on the settings page.
All outbound calls made while signed into that account will use the caller ID of the primary number.
If you would rather handle calls made to each of the two inbound numbers independently, then you would need to port the second number into a different Google account.
Calls made to the inbound GV number(s) are then simultaneously forwarded to all of the
forwarding telephone numbers you add to your list, on this page:
https://www.google.com/voice#phonesYou can have up to six forwarding numbers. You can also forward inbound calls to an OBi device by check-marking the Google Chat forwarding destination.