Tikz - Overlaping border on adjoining nodes

Question

I am having trouble obtaining a single border between two adjoining node shapes with tikz. When I use a thick border, it leads to an overlapping double border at the connection of two nodes.
How would one avoid this?

I currently try using the positioning library as well as the chain library.

I currently have this

Which I produce with the following code.

\documentclass{standalone} % standalone 
\usepackage{graphicx}
\usepackage[usenames,dvipsnames,svgnames,table]{xcolor}
\usepackage{tikz}
\usetikzlibrary{chains,shapes,positioning}

\begin{document}

\tikzstyle{b1} = [rectangle,
                  fill = red!50,
                  draw = black, thick,
                  fill opacity = 0.5, text opacity = 1]
\tikzstyle{b2} = [rectangle,
                  fill = blue!50,
                  draw = black, thick,
                  fill opacity = 0.5, text opacity = 1]               

\begin{tikzpicture}[auto,node distance=1cm and 0cm,every node/.style={font=\small}]

\node(A)[b1] at (0,0) {A};
\node(B)[b2,right=of A]{B};

\end{tikzpicture}

\begin{tikzpicture}[start chain,node distance=0mm,every node/.style={font=\small}]

\node[on chain,b1] {A};
\node[on chain,b2] {B};

\end{tikzpicture}

\end{document}

Answer 1

I will extend my comment to answer:

\documentclass[border=3mm]{standalone} % standalone
\usepackage{tikz}

\usetikzlibrary{chains,shapes,positioning}
\tikzset{
every node/.style = {rectangle, draw=black, thick,
                     fill opacity = 0.5, text opacity = 1,
                     font=\small, 
                    outer sep=0pt,% <-- this eliminate your problem
                     },
b1/.style = {fill=red!50},
b2/.style = {fill=blue!50}
        }

    \begin{document}
\begin{tikzpicture}[auto,node distance=1cm and 0cm]
    \node(A)[b1] {A};
    \node(B)[b2,right=of A]{B};
\end{tikzpicture}

\begin{tikzpicture}[start chain=going right,node distance=0mm]
    \node[on chain,b1] {A};
    \node[on chain,b2] {B};
\end{tikzpicture}
    \end{document}

Above code (which is based on slightly shorten your MWE) generate: