rest in two voices

[Juergen Goebbels]

Example:

 

\version "2.18.0"

\new Staff <<

\clef bass

\new Voice = "first" {

\voiceOne

      b8 r8 b r 

}

\new Voice = "second" {

\voiceTwo

      a8 r g r

}

>> 

 

I have two questions:

 

-          How can the rest  in the two voices reduced to one rest in the middle of the staff?

-          Can the pitch “a” appear before the pitch “b” ?         

 

Thanks in Advance

 

Juergen