\renewcommand\twofuncs[2]{%
  \HCode{%
    <munderover>%
      <mo>&rightrightarrows;</mo>%
      <mstyle style="font-size: 0.6em;"><mrow>}#2\HCode{</mrow></mstyle>%
      <mstyle style="font-size: 0.6em;"><mrow>}#1\HCode{</mrow></mstyle>%
    </munderover>%
  }%
}

\renewcommand\customstackrel[3][]{
  \HCode{%
    <munderover>%
      <mo>}#3\HCode{</mo>%
      <mstyle style="font-size: 0.6em;"><mrow>}#1\HCode{</mrow></mstyle>%
      <mstyle style="font-size: 0.6em;"><mrow>}#2\HCode{</mrow></mstyle>%
    </munderover>%
  }%
}

\renewcommand\funcslr[2]{%
  \HCode{%
    <munderover>%
      <mo>&rightleftarrows;</mo>%
      <mstyle style="font-size: 0.6em;"><mrow>}#2\HCode{</mrow></mstyle>%
      <mstyle style="font-size: 0.6em;"><mrow>}#1\HCode{</mrow></mstyle>%
    </munderover>%
  }%
}