Besides just using two different TikZ pictures that are separated by \\, you can use the manual added node note-1 to shift the whole second tree down.
You can also use an already defined node of the Tree but you need to re-establish its coordinates by the help of an auxiliary coordinate (Code 2):
% somewhere in the tree:
[. \node (lowest node) {1-2-2-1-1}; ]
% outside of tree:
\coordinate (aux1) at (lowest node);
% The coordinate aux1 can now be used for placement, e.g.
% below=of toptree |- aux1
The |- makes sure that the 2 is placed under the 1.
Code 1
\documentclass[tikz]{standalone}
\usepackage{tikz,tikz-qtree,tikz-qtree-compat}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\Tree
[.\node (toptree) {1};
[.1-1 ]
[.\node (1) {1-2}; ]
]
\node [below = 0.5cm of 1.south] (note-1) {la la la};
\draw[-] (note-1.north) -- (1.south);
\begin{scope}[every tree node/.append style={below=of toptree |- note-1.south}]
\Tree
[.2
[.2-1 ]
[.2-2 ]
]
\end{scope}
\end{tikzpicture}
\end{document}
Code 2
\documentclass[tikz]{standalone}
\usepackage{tikz,tikz-qtree,tikz-qtree-compat}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\Tree
[.\node (toptree) {1};
[.1-1 ]
[.1-2
[.1-2-1
[.1-2-2-1
[. \node (lowest node) {1-2-2-1-1}; ]
]
]
]
]
\coordinate (aux1) at (lowest node);
\begin{scope}[opacity=.5,every tree node/.append style={below=of toptree |- lowest node}]
\Tree
[.2
[.2-1 ]
[.2-2 ]
]
\end{scope}
\begin{scope}[every tree node/.append style={below=of toptree |- aux1}]
\Tree
[.2
[.2-1 ]
[.2-2 ]
]
\end{scope}
\end{tikzpicture}
\end{document}
Output

tikzpictureenvironments, and add a line break between them? For example,\begin{tikzpicture} <codes for 1st tree> \end{tikzpicture} \\ \begin{tikzpicture} <codes for 2nd tree> \end{tikzpicture}– Herr K. Apr 03 '13 at 20:59