Boxes placement in tcolorbox

Obviously, you would use the \tcbox macro version for your boxes, rather than the environment form. Calculation of the stackgap is shown automated in this MWE. (Note: this example works starting with V2.30 of tcolorbox).

\documentclass{article}
\usepackage{stackengine}
\usepackage{tcolorbox}
\begin{document}

\def\boxone{\tcbox[nobeforeafter]{This is a test}}
\def\boxtwo{\tcbox[nobeforeafter]{Another test}}
\def\boxthree{\tcbox[nobeforeafter,minipage,width=0.7in]{this is a very long box}}

\stackon[\heightof{\boxthree}-\heightof{\boxone}-\heightof{\boxtwo}]%
  {\boxtwo}{\boxone}~\boxthree

\end{document}

Taking a similar tack to Steven Segletes's answer but using the xcoffins package, which allows us to pick points on the various boxes to line up:

\documentclass{article}
\usepackage{tcolorbox}

\usepackage{xcoffins}
\NewCoffin{\CoffinA}
\NewCoffin{\CoffinB}
\NewCoffin{\CoffinC}
\begin{document}
% Set up content in three coffins
\SetHorizontalCoffin{\CoffinA}{%
  \begin{tcolorbox}[width = 1 in]
    This is a \textbf{tcolorbox}.%
  \end{tcolorbox}
}%
\SetHorizontalCoffin{\CoffinB}{%
  \begin{tcolorbox}[width = 1 in]
    This is a \textbf{tcolorbox}.%
  \end{tcolorbox}
}%
\SetHorizontalCoffin{\CoffinC}{\rule{1 in}{2.5 in}}%
% Join the first small coffin onto the big one at the top
\JoinCoffins{\CoffinC}[t,l]{\CoffinA}[t,r](-0.1 in, 0 in)%
% Join the second small coffin onto the big one at the bottom
\JoinCoffins{\CoffinC}[b,\CoffinC-l]{\CoffinB}[b,r](-0.1 in, 0 in)%
\TypesetCoffin{\CoffinC}

\end{document}

Here, I've added a small gap between the coffins (it's negative as I've done everything relative to the large item, \CoffinC.)

The 'coffin' system works by having 'handles' on the boxes it creates: we can address those by letters, e.g. t for 'top', and also refer to points on coffins we've already combined. This allows relatively easy construction of complex box relationships.