I would simply use \discretionary for this and define a macro to increase comfort.
\documentclass{article}
\usepackage{xspace}% see http://ctan.org/pkg/xspace
\usepackage[textwidth=2.78in,textheight=0.9in]{geometry}% for demonstration only
\usepackage{showframe}% for demonstration only
\newcommand*{\AB}{\discretionary{A}{B}{AB}\xspace}
\begin{document}
\AB vs. some text, some text, some text, some text, some text, some text, some
text \AB and some text.
\AB vs. some text, some text, some text, some text, some text, some text, some
text \AB and some text.
\end{document}
I've used package showframe to demonstrate the width and height of the text area at my example (suggestion from Werner). I've used very small width and height to demonstrate line break and page break.
Here are snapshots of the relevant area of page 1 and 2:

If you want, you may define a more general command:
\documentclass{article}
\usepackage[textwidth=2.78in,textheight=0.9in]{geometry}% for demonstration only
\usepackage{showframe}% for demonstration only
\newcommand*{\breakchars}[2]{\discretionary{#1}{#2}{#1#2}}
\begin{document}
\breakchars AB vs. some text, some text, some text, some text, some text, some
text, some text \breakchars AB and some text.
\breakchars AB vs. some text, some text, some text, some text, some text, some
text, some text \breakchars AB and some text.
\breakchars VA vs. some text, some text, some text, some text, some text, some
text, some text \breakchars VA and some text.
\end{document}
You don't need argument braces at this case, because without the first and second non-space character (letter or other) will be used to be the first and second argument.
The example with VA was made to show, that kerning still works with this suggestion.
A\discretionary{}{}{}Bis simpler, though. – egreg Nov 19 '11 at 10:16VA \discretionary{V}{A}{VA} V\discretionary{}{}{}A. The first and the second are kerned, at the third the kerning is missing. – Schweinebacke Nov 19 '11 at 10:23