No documented synergistic effects exist, because the combination has not been studied. The question imagines two practices working together, yet there is no body of research pairing Reiki with transcranial direct current stimulation, and inventing results for that pairing would not be honest. What can be done is to look at each practice on its own terms and explain why the proposed teaming is speculative.
Take tDCS first. It is a real neuromodulation method that passes a weak electrical current across the scalp to nudge the excitability of brain regions. Its evidence is genuinely mixed. In clinical settings, with trained staff, it appears moderately helpful for some people with depression, while a large home-use trial found no advantage over a sham device. Researchers are still working out who benefits, how to target it, and how durable any effect is. So even the established half of this pairing carries real uncertainty.
Reiki is the other half, and its situation is different. There is no demonstrated energetic mechanism behind it. What people reliably get from a session is relaxation, a sense of being cared for, and the calming effect of quiet attention and gentle contact. Those are worthwhile, but they are explained by ordinary psychology and physiology, not by an energy field that could interact with an electrical current.
Put the two together and the idea of synergy starts to look thin. One could hypothesize that the relaxation from a Reiki session makes a person calmer and more comfortable during a tDCS appointment, which might marginally improve how tolerable the procedure feels. That is a reasonable comfort-and-context claim, and it is about as far as the reasoning can responsibly go. There is no basis for saying that Reiki primes neural plasticity, aligns energetic and electrical dimensions, or amplifies the stimulation itself. Those phrases describe a mechanism that has not been shown to exist.
It is also worth being clear about safety and framing. tDCS is a medical-style intervention that should be used under qualified supervision and within trials or approved protocols. Reiki, whatever comfort it offers, does not change what the stimulation does to the brain. Anyone drawn to this combination is better served by viewing relaxation practices as a way to feel more at ease around a treatment, never as a treatment substitute, and by keeping decisions about neuromodulation firmly with clinicians. The pairing is an interesting thought experiment. As of now it is exactly that, and nothing has been measured to make it more.