expl3 replace \tex_lowercase:D for colon parsing

You can exploit \use:x

\documentclass{article}
\usepackage{expl3,xparse}

\ExplSyntaxOn

\cs_new_nopar:Npn \__example_point:nn #1#2
  {
    Example~with~#1~and~#2.
  }

\use:x
  {
    \cs_new_nopar:Npn \exp_not:N \__example_point:w ##1 \c_colon_str ##2 \exp_not:N \q_stop
  }
  {
    \__example_point:nn {#1}{#2}
  }

\cs_new_nopar:Npn \__example_point:n #1
  {
    \__example_point:w #1 \q_stop
  }

\NewDocumentCommand{\myexample}{ m }{ \__example_point:n {#1} }

\ExplSyntaxOff


\begin{document}

\myexample{34.2:47.9}

\end{document}

Note \cs_new…, which is better practice than \cs_set….

This also works:

\ExplSyntaxOn

\cs_new_nopar:Npn \__example_point:nn #1#2
  {
    Example~with~#1~and~#2.
  }

\tl_set:Nx \l_tmpa_tl { ##1 \c_colon_str ##2 }
\exp_last_unbraced:NNV \cs_new_nopar:Npn \__example_point:w \l_tmpa_tl \q_stop
  {
    \__example_point:nn {#1}{#2}
  }

\cs_new_nopar:Npn \__example_point:n #1
  {
    \__example_point:w #1 \q_stop
  }

\NewDocumentCommand{\myexample}{ m }{ \__example_point:n {#1} }

\ExplSyntaxOff

You could use \seq_set_split:Nnn like this:

\documentclass{article}
\usepackage{expl3,xparse}

\ExplSyntaxOn


\cs_set_nopar:Npn \__example_point:nn #1#2
  {
    Example~with~#1~and~#2.
  }

\cs_generate_variant:Nn \seq_set_split:Nnn {NVn}

\cs_set_nopar:Npn \__example_point:n #1
  {
    \seq_set_split:NVn \l_tmpa_seq \c_colon_str {#1}
    \__example_point:nn {\seq_item:Nn\l_tmpa_seq {1}}{\seq_item:Nn\l_tmpa_seq {2}}
  }


\DeclareDocumentCommand{\myexample}{ m }{ \__example_point:n {#1} }

\ExplSyntaxOff


\begin{document}

\myexample{34.2:47.9}

\end{document}