I use a Sinewave Revolution.
Confirming what others have said, you wire the light and Sinewave in parallel. When you want max power into the Sinewave, turn off the light. When you want the lights on, you do not need to unplug the Sinewave from the wiring. No other plugging/unplugging or switches is needed.
My GPS (a non-cycling model) does not play well with the Sinewave directly. I need to run the power from the Sinewave into a battery pack that functions as a pass through cache battery, then I can plug my GPS into that battery pack. Most battery packs do not accept a charge and also charge devices simultaneously (pass through cache battery), I use one that does. I use a larger powerbank for that purpose, I bought it for touring so it supplies much more power than just my GPS, larger than you would need or want for brevets.
I generally tell people that they should assume they will get no charging out of their Sinewave (or other charger) when their lights are on, but I was surprised that I got some power out of the Sinewave.
My lighting is a B&M IQ-XS and have used a Secula but now am using a Spanninga Pixio for taillight. With the Secula and IQ-XS turned off, during an exercise ride on flat ground I averaged about 2.52 watts coming out of the Sinewave (about half an amp), and into my powerbank. That is only measured while rolling, not while stopped at stop lights, etc. And while riding (again, flat ground) measured during an exercise ride 0.60 watts out of the Sinewave with both my IQ-XS and Secula turned on. So, as a rule of thumb, on flat ground my Sinewave only produced one quarter as much power with lights on as with lights off. Both of these tests were a bit over an hour.
I can't tell if my lights are any dimmer or not if I have plugged anything into the Sinewave. But since I expect so little power out of the Sinewave when the lights are on, I usually unplug the USB cable when the lights are on. Perhaps my standlight charges up better when not using the Sinewave to charge anything if more power is available to the lights?
I bought the Sinewave for touring, I think it is the best for waterproofness of all options, but it lacks any form of pass through cache battery, so when you stop, power out of it will stop.
I assume your IQ-X draws more power than my IQ-XS because it is a brighter light, so maybe you would get less power out of the Sinewave when the light is on than I do.
And my measurements were on fairly flat ground, not intermittently going slow and then going fast up and down hills. Where I measured 2.52 watts (lights off) on fairly flat ground, I would assume on more uneven ground with fluctuating speeds to get no more than 2 watts on average, maybe even less. Practically speaking, I assume I am getting zero power out of the Sinewave when I am using my granny chainring on my triple crank up steeper hills.
I did these measurements with a SP PV-8 hub, 700c wheel.