Yes, a single neuron can release multiple neurotransmitters.
For example, a single inhibitory spinal inteuron can release both GABA
and glycine from the same vesicle, which act seperately on GABA and
glycine receptors in the same postsynaptic element. See for example
this beautiful paper:
Jonas P, Bischofberger J, Sandkuhler J. Corelease of two fast
neurotransmitters at a central synapse.
Science. 1998 Jul 17;281(5375):419-24.
Furthermore, many transmitters are co-released with ATP, which itself
could be considered a transmitter in some circumstances.
Also, many pre-synapses contain both "small synaptic vesicles" (SSVs)
and "large dense-core vesicles" (LDCVs). The former contain the usual
small amino acid or catecholamine neurotransmitters, whereas the latter
often contain peptide neurotransmitters such as opiods like enkephalin
The two types of vesicles (hence transmitter types) are packaged and
released differently, usually in a manner that depends on the calcium
load in the presynaptic terminal. For example, SSVs (containing e.g.,
ACh, glutamate, GABA) are released after single spikes, whereas high
frequency trains are required to mobilize and release LDCVs containing
peptides. Much of this work was done in the classical system of
Aplysia, but I bet it applies to mammalian synsapses as well. See
especially Whim & Lloyd (1989) PNAS 86:9034-9038.