A quick way of showing how the two structures can be positioned relative to each other in a single Graphics3D is as follows:
With[{rMax = 500},
Manipulate[
Graphics3D[
{First@ChemicalData["Acetone", "MoleculePlot"],
GeometricTransformation[
First@ChemicalData["Chloroform", "MoleculePlot"],
Composition[
TranslationTransform[{displacementX, displacementY,
displacementZ}],
RotationTransform[
rotationAngle, {Cos[axisAzimuth] Sin[axisPolar],
Sin[axisAzimuth] Sin[axisPolar], Cos[axisPolar]}]]]},
PlotRange -> {{-rMax, rMax}, {-rMax, rMax}, {-rMax, rMax}}
],
{{displacementX, 0}, -rMax, rMax}, {{displacementY, 0}, -rMax,
rMax}, {{displacementZ, 0}, -rMax, rMax}, {rotationAngle, 0,
2 Pi},
{axisPolar, 0, 2 Pi}, {axisAzimuth, 0, 2 Pi}
]
]

The Manipulate is just to let you do some manual re-positioning. You can eventually get better results by doing pre-calculated shifts and rotations based on the exact locations of the atoms. But I'm focusing here on the basic ingredients. For the display, this involves mainly the use of First to extract the contents of the Graphics3D of the molecule, then the translation and rotation using GeometricTransformation with a Composition of the former two operations.
The variable rMax sets the plot range and the maximum displacement of the chloroform molecule relative to the acetone.
FullFormof the molecular plots, you may come up with a one-off solution. Some assembly may be required. – DavidC Apr 07 '13 at 20:11