Hebbian Learning - Understanding Simultaneous Firing
You basically have 2 options:


Manually fire both neurons together that you want to pair - do this as many
times as needed to pair them.  After learning, it should be
sufficient to fire only one neuron for the second neuron to fire as
desired.
Assign a starting weight to the connection between the
neurons such that firing one will trigger the other.  This is
basically shortcutting the above learning process.


Normally, the learning process in neural networks is based on multiple simultaneous inputs - not just one.  The network learns the statistical probability of input neurons firing together, and is eventually able to predict them without them firing.  So a test case of a single neuron firing can be used after learning is complete (the first option) or when you restore a previously saved matrix of a trained network (the second option).