documentclass{article}

usepackage{xparse}

newcommand{eset}[1]{mathchoice{left {#1right }}{{#1}}{{#1}}{{#1}}}

ExplSyntaxOn
NewDocumentCommand lip { D||{A} m D||{B} m } {
  #1^#2, #3^#4
}
ExplSyntaxOff


% How can I define slip without the xparse boilerplate?
ExplSyntaxOn
NewDocumentCommand slip {D||{A} m D||{B} m } {
  eset{lip |#1| {#2} |#3| {#4}}
}
ExplSyntaxOff

begin{document}

[lip |a| {1} |b| {2}]

[slip |a| {1} |b| {2}]

end{document}