expl3 macro to expandably iterate on a list and apply a macro on each pair of items

Quite similar to the implementation in my answer to your other question, but this one takes two arguments, so we need to add two q_recursion_tail. Also another intermediary macro is used to insert a prg_do_nothing: in the second item to avoid losing spaces and braces.

documentclass{article}
usepackage{xparse}
ExplSyntaxOn
NewExpandableDocumentCommand multiapplycouple { m +m }
  { mbc_multiapply_pair:Nn #1 {#2} }
cs_new:Npn mbc_multiapply_pair:Nn #1 #2
  {
    __mbc_multiapply_pair:nNw { 0 } #1
      | prg_do_nothing: #2 | q_recursion_tail | q_recursion_stop
  }
cs_new:Npn __mbc_multiapply_pair:nNw #1 #2 | #3 |
  { __mbc_multiapply_pair:nNww {#1} #2 | #3 | prg_do_nothing: }
cs_new:Npn __mbc_multiapply_pair:nNww #1 #2 | #3 | #4 |
  { __mbc_multiapply_pair:oofN {#3} {#4} { int_eval:n {#1+1} } #2 }
cs_new:Npn __mbc_multiapply_pair:nnnN #1 #2 #3 #4
  {
    quark_if_recursion_tail_stop:n {#2}
    quark_if_recursion_tail_stop:n {#1}
    exp_not:e { exp_not:N #4 {#3} { tl_trim_spaces:n {#1} } { tl_trim_spaces:n {#2} } }
    __mbc_multiapply_pair:nNww {#3} #4 | prg_do_nothing: #2 | prg_do_nothing:
  }
cs_generate_variant:Nn __mbc_multiapply_pair:nnnN { oof }
ExplSyntaxOff

newcommanddecocouple[3]{Item #1: (#2)[#3]quad}

begin{document}
multiapplycoupledecocouple{ 1 | 12 | 123 | 1234 }
end{document}