Well I don't understand exactly what you what to do with the table, or if you are understanding well how to build a table in LaTeX.
According to what I understood from your table, and from your MWE, I would write it in this way
\documentclass{article}
\usepackage{siunitx,booktabs,caption}
\newcommand{\splitcell}[1]{%
\begin{tabular}{@{}c@{}}\strut#1\strut\end{tabular}%
}
\begin{document}
\begin{table}[htp]
\centering
\caption{The table caption}
\begin{tabular}{S[table-format=2.0] S[table-format=1.0] S[table-format=1.6] S[table-format=1.4] S[table-format=1.3]}
\toprule
{$t$ (\si{\nano\meter})} & {$W$ (\si{\micro\meter})} & \multicolumn{3}{c}{My results} \\
\cmidrule{3-5}
& & {Re} & {\splitcell{Theor.\\Overlap\\factor}} & {\splitcell{Theor.\\MPA\\(\si{dB/mm})}} \\
\midrule
25 & 8 & 1.447413 & 0.9627 & 2.128 \\
& 4 & 1.446128 & 0.9538 & 1.20 \\
\midrule
31 & 8 & 1.448499 & 0.9377 & 4.196 \\
& 4 & 1.446889 & 0.9571 & 2.78 \\
\bottomrule
& & \multicolumn{3}{c}{Her results} \\
\cmidrule{3-5} % 6-8
& & {Re} & {\splitcell{Theor.\\Overlap\\factor}} & {\splitcell{Theor.\\MPA\\(\si{dB/mm})}} \\
\midrule
25 & 8 & 1.447413 & 0.9627 & 2.128 \\
& 4 & 1.446128 & 0.9538 & 1.20 \\
\midrule
31 & 8 & 1.448499 & 0.9377 & 4.196 \\
& 4 & 1.446889 & 0.9571 & 2.78 \\
\bottomrule
\end{tabular}
\end{table}
\end{document}
Notice that I widened the rows for align them and in this way make easier to see how the table was build. So as you can see, the table is right with just 5 columns. If you need more, then you must to add them in the same line where you define the tabular environment. But don't confuse the rows with the columns.
Update
Finally you asked if it's possible to get the results of her near to yours instead of beneath them?
Sure it's possible, but it depends not of LaTeXbut you to define correctly the table. The example you showed has 5 columns, if you need the results to be contiguous to yours, then you must change the dimensions of the table from the beginning.
Something like this:
\documentclass{article}
\usepackage{siunitx,booktabs,caption}
\newcommand{\splitcell}[1]{%
\begin{tabular}{@{}c@{}}\strut#1\strut\end{tabular}%
}
\begin{document}
\begin{table}[htp]
\centering
\caption{The table caption}
\begin{tabular}{
S[table-format=2.0] % Column 1
S[table-format=1.0] % Column 2
S[table-format=1.6] % Column 3
S[table-format=1.4] % Column 4
S[table-format=1.3] % Column 5
S[table-format=1.6] % Column 6 same width than 3
S[table-format=1.4] % Column 7 same width than 4
S[table-format=1.3] % Column 8 same width than 5
}
\toprule
{$t$ (\si{\nano\meter})} & {$W$ (\si{\micro\meter})} & \multicolumn{3}{c}{My results} & \multicolumn{3}{c}{Her results} \\
\cmidrule{3-5} \cmidrule{6-8}
& & {Re} & {\splitcell{Theor.\\Overlap\\factor}} & {\splitcell{Theor.\\MPA\\(\si{dB/mm})}} & {Re} & {\splitcell{Theor.\\Overlap\\factor}} & {\splitcell{Theor.\\MPA\\(\si{dB/mm})}}\\
\midrule
25 & 8 & 1.447413 & 0.9627 & 2.128 & 1.447413 & 0.9627 & 2.128\\
& 4 & 1.446128 & 0.9538 & 1.20 & 1.446128 & 0.9538 & 1.20\\
\midrule
31 & 8 & 1.448499 & 0.9377 & 4.196 & 1.448499 & 0.9377 & 4.196\\
& 4 & 1.446889 & 0.9571 & 2.78 & 1.446889 & 0.9571 & 2.78\\
\bottomrule
\end{tabular}
\end{table}
\end{document}
You need to add in the line \begin{tabular}{... the last 3 columns for get the same width of column, this is: S[table-format=1.6] S[table-format=1.4] S[table-format=1.3].
The second part of the trick is to fill the rows with the content of those three columns.
In the line 29 you can add now \cmidrule{6-8} without errors.
In your MWE the data in the columns 6 and 8 are the same and if this is true would be redundant since you only need to add (I think) the column of her results. In that case the table is narrower and less complex.
\midruleyou wrote\cmidrule, that's the error. Please try to read the information of the bugs when you compile your documents, there is very useful information. – Aradnix Aug 14 '14 at 17:12\cmidrulecommand in fact, but it was because the definition was wrong. In the line 20 you create a rule between the columns 3 and 5 and was right. Bun in the line 42 you try to create another rule between the columns 6 and 8 in a table of 5 columns, that's the mistake. Also the rowHer resultsis misaligned, you need to add in the line 33 a couple of ampersands (&) before the\multicolumn{3}{c}{Her results}. – Aradnix Aug 14 '14 at 17:24tabularenvironment:\begin{tabular}{columns}You defined 5 columns S from the
– Aradnix Aug 14 '14 at 17:45siunitxpackage. If you need more, then you need to add those columns there. But sincerely I don't think you need do that.