The mechanism of action of L-Theanine is not quite well understood and at some degree recent studies find themselves in contradictory with the results, nevertheless I think the essencial is this
Glutamate is a powerful excitatory neurotransmitter. It plays an important role in cognitive functions such as learning and memory due to its role in synaptic plasticity.
blocks the reuptake of glutamine and glutamate
and thus it will increase the concentration of glutamate in the synaptic cleft,
prolonging the effect
The mechanism of action of caffeine at the Central Nervous System can be summed up to one thing
In general, adenosine has an inhibitory effect in the central nervous system (CNS)
Caffeine demonstrates antagonism of all 4 adenosine receptor subtypes (A1, A2a, A2b, A3) in the central nervous system
which means that caffeine will have an excitatory effect on the central nervous system.
They are both stimulants of the CNS, however when combining the two we have an interesting conclusion by the authors in this article so that
l-theanine can counteract the stimulatory effect of caffeine. In rats, after caffeine administration intravenously with theanine at the same dose, the stimulant effect of caffeine was blunted. Whereas given by normal dose of caffeine with a smaller dose of theanine administration resulted in excitatory effects of caffeine, which suggested a dose specific dual activity of theanine
In summary it depends on the dose.