In order to make the five columns equally wide, you have several options. The following MWE includes two of them. In both examples, I made the text in the multicolumn narrower by using a linebreak or by adding another shared column header.
\documentclass{article}
\usepackage{booktabs}
\usepackage{geometry}
\begin{document}
\begin{table}\centering
\begin{tabular}{l c c c c c c c c c c}
\toprule
& \multicolumn{5}{c}{Evaluate on}\\
& \multicolumn{5}{c}{complete ground truth} & \multicolumn{5}{c}{Evaluate on truncated} \\
\cmidrule(l{2pt}r{2pt}){2-6} \cmidrule(l{2pt}r{2pt}){7-11}
& 0s & 1s & 2s & 3s & 4s & 0s & 1s & 2s & 3s & 4s \\
\midrule
Multi-Task Net & 1 & 2 & 3 & 4 & 5 & 0s & 1s & 2s & 3s & 4s \\
CNN & 71.3 & & & & & & 23.9 & 30.3 & & \\
Keypoints & 55.6 & & & & & & 10.0 & 15.2 & & \\
JAAD & 68.3 & & & & & & 18.3 & 24.9 & & \\
\midrule
CNN LSTM & 68.3 & & & & & & 18.3 & 24.9 & & \\
Keypoints LSTM & 68.3 & & & & & & 18.3 & 24.9 & & \\
3D CNN & 68.3 & & & & & & 18.3 & 24.9 & & \\
\bottomrule
\end{tabular}
\caption{Foo matter}
\end{table}
\begin{table}\centering
\begin{tabular}{l c c c c c c c c c c}
\toprule
& \multicolumn{10}{c}{Evaluate on}\\ \cmidrule(l{2pt}r{2pt}){2-11}
& \multicolumn{5}{c}{complete ground truth} & \multicolumn{5}{c}{truncated} \\
\cmidrule(l{2pt}r{2pt}){2-6} \cmidrule(l{2pt}r{2pt}){7-11}
& 0s & 1s & 2s & 3s & 4s & 0s & 1s & 2s & 3s & 4s \\
\midrule
Multi-Task Net & 1 & 2 & 3 & 4 & 5 & 0s & 1s & 2s & 3s & 4s \\
CNN & 71.3 & & & & & & 23.9 & 30.3 & & \\
Keypoints & 55.6 & & & & & & 10.0 & 15.2 & & \\
JAAD & 68.3 & & & & & & 18.3 & 24.9 & & \\
\midrule
CNN LSTM & 68.3 & & & & & & 18.3 & 24.9 & & \\
Keypoints LSTM & 68.3 & & & & & & 18.3 & 24.9 & & \\
3D CNN & 68.3 & & & & & & 18.3 & 24.9 & & \\
\bottomrule
\end{tabular}
\caption{Foo matter}
\end{table}
\end{document}

tabularxandtabularypackages? You don't seem to use them in your code. – leandriis Jun 05 '19 at 18:27tablesinstead. (From the tag description: {tables} is about the tabular environment and related packages [...]) – leandriis Jun 05 '19 at 18:33