\begin{filecontents}{a.tex}
\tag{1.1}{this is 1.1 from file a.}
\tag{1.2}{this is 1.2 from file a.}
\end{filecontents}
\begin{filecontents}{b.tex}
\tag{0.1}{this is 0.1 from file b.}
\tag{1.1}{this is 1.1 from file b.}
\tag{1.2}{this is 1.2 from file b.}
\end{filecontents}
\documentclass{article}
\makeatletter
\newcommand{\print}[2][]{%
\if\relax\detokenize{#1}\relax
\print@all{#2}%
\else
\print@one{#1}{#2}%
\fi
}
\newcommand{\taginput}[1]{%
\ifx\taginput@files\@empty
\gdef\taginput@files{#1}%
\else
\g@addto@macro\taginput@files{,#1}%
\fi
\def\taginput@current{#1}%
\input{#1}%
}
\let\taginput@files\@empty
\newcommand{\tag}[2]{%
\global\@namedef{taginput@\taginput@current @#1}{#2}%
}
\newcommand{\print@all}[1]{%
\begingroup\edef\x{\endgroup
\noexpand\@for\noexpand\next:=\taginput@files\noexpand\do
}\x{\print@one{\next}{#1}}%
}
\newcommand{\print@one}[2]{%
\@nameuse{taginput@#1@#2}\par
}
\makeatother
\begin{document}
\tag{0.1}{this is 0.1 from file main.}
\taginput{a}
\tag{1.1}{this is 1.1 from file main.}
\taginput{b}
\tag{0.1}{this is 0.1-2 from file main.}
\print[a]{1.1} % result 1 line (1.1 from a)
\print[b]{1.2} % result 1 line (1.2 from b)
\bigskip
\print{1.1} % result 3 lines (1.1 from a, 1.1 from main, 1.1 from b)
\bigskip
\print{1.2} % result 2 lines (1.2 from a, 1.2 from b)
\bigskip
\print{0.1} % result 3 line (0.1 from main, 0.1 from b, 0.1-2 from main)
\end{document}
