digitizing two polygons

Hello!
I have some questions!

for example, I have two polygons having a common boundary,
so I'm gonna digitizing them in Autocad R13.
As far as I know, I should digitize the common boundary twice to make
two polygons.

Is it true?
If not, can I get two polygons by digitizing the common boundary just once?

Thank you in advance!

Sungwon Choi