Tie multiple voice music contained in variable

[Gianmaria Lari]

The following code containing a tie between variables, compiles and works perfectly:

\version "2.21.0"
pa = {e'1}

{ \pa~\pa }  %here is the tie!

Is there any way to do the same in case the variable contains two voices like in the following code?

\version "2.21.0"
pa = {<< {e'1}\\{c'1}>>}
{ \pa~ \pa } %this does not compile

Thank you, g.