You can use this circuit to give auto trigger for your secondary flash light. You don’t have to worry about wiring for the control, because using this circuit your secondary flash will be automati8cally triggered when it receive the light signal from the primary flash. Look at the circuit’s schematic diagram below.
The inductor 68mH is connected in parallel with the solar cell to prevent the false trigger by slowly changing environment light. Bright daylight won’t trigger your slave flash lamp because the output will be shorted by the inductor. Only fast luminance change caused by your primary flash lighting will trigger your secondary flash light. The solar cell should be 100mm2 at minimum (about 1cm2), and you can easily find on many electronics part store, and sure you can use a larger one for better sensitivity. For the inductor, because it is relatively huge, you can use a radio frequency choke (RFC) that usually used in AM radio transmitter circuit, but any inductor with low enough resistance could be used.