1. this is my first paragraph in first item.
   nextline, same paragraph

   this is my second paragraph in first item.
   nextline, same paragraph

2. this is my first paragraph in second item.
   nextline, same paragraph

1. this is my first paragraph in first item.
   nextline, same paragraph

       this is my second paragraph in first item.
   nextline, same paragraph

2. this is my first paragraph in second item.
   nextline, same paragraph

documentclass{article}

begin{document}

begin{enumerate}
  item
   this is my first paragraph in first \item.
   nextline, same paragraph

   noindent
   this is my second paragraph in first \item.
   nextline, same paragraph

  item
   this is my first paragraph in second \item.
   nextline, same paragraph

   noindent
   this is my second paragraph in second \item.
   nextline, same paragraph

end{enumerate}

end{document}

setlength{parindent}{0pt}