If you're following this route, I would suggest using the \label-\ref system and create your own macros to handle the labelling and cross-referencing. Here's one take on it that uses refcount to pass references as values to the footnote mechanism (which uses counters):

\documentclass{article}
\usepackage{refcount}% http://ctan.org/pkg/refcount
\newcounter{fncntr}
\newcommand{\fnmark}[1]{\refstepcounter{fncntr}\label{#1}\footnotemark[\getrefnumber{#1}]}
\newcommand{\fntext}[2]{\footnotetext[\getrefnumber{#1}]{#2}}
\begin{document}
Some text\fnmark{first-fn} and some more\fnmark{second-fn} text,
and finally\fnmark{last-fn} this.
\fntext{first-fn}{First footnote.}
\fntext{second-fn}{Second footnote.}
\fntext{last-fn}{Last footnote.}
\end{document}
Each \fnmark{<label>} is supplied with a label, which is then synchronized with a \fntext{<label>}{<text>} somewhere else in the document. Since this uses the .aux file to store references, you're going to have to compile at least twice to start with (all footnotes will be 0, as per refcount's \setrefcountdefault{0}). However, subsequent runs will only require a single compile, until new footnotes are introduced, or the references change.
sepfootnotespackage – cgnieder Nov 26 '13 at 11:23