When two excitatory postsynaptic potentials (epsps) occur at a single synapse so rapidly in succession that the postsynaptic neuron's membrane potential has not returned to the resting potential before the second epsp arrives, the epsps add together, producing

As mentioned the two EPSPs processes combine together producing a process called temporal summation The first procedure is fast in process where as the second procedure awaits the arrival of second EPSPs unlike in spatial summation, where the processes occurs in different phases.