I have a valve in my heating system that I don't understand. I have searched on the internet but I haven't been able to find any information on it. What I do know is that the valve's purpose is that of maintaining the pressure in the system, but I don't know why there are two controls or what each of the two controls do. All other pressure control vales that I've seen have just a single control. The two controls are marked in the photo. There is a hand operated valve ( similar to a tap ) on the bottom and there is a screwdriver operated control on the top with a + and - on it, but turning it doesn't seem to do much if anything and it doesn't appear to have any stop it just seems to keep turning but I have only gone 10 or 15 turns before I turned it back, maybe it's more.
It's called a pressure reducing valve (PRV)
That's a PRV with a shut off and set point. The knob on the bottom is the shut off and the bigger knob with the screw is the set point. Now that you've adjusted it with pressure in the system you may have lowered it and you may need to remove some water to determine the set point. From the picture it looks like you're set at about 2 bar which is about 30 PSI in my neck of the woods 30 PSI on heating system is the upper limit and close to it or much beyond it will cause the pressure relief valve to pop.
What the pressure does
I'm not going to do the exact math but one BAR will push water up about 10 meters. So if the highest radiator in your house is around ten meters high from the point of measurement you would want your pressure-reducing valve set to maybe 1.2 BAR to compensate for errors.
What to do
If the only thing you did was turn the bottom one then no harm done. If you did adjust the top one you're going to need to remove some water but before you do that you're going to need to determine what the correct pressure for your system is. Often on that type of pressure reducing valve the screw on the top is what actually makes the adjustment and the black plastic is just a cap covering the housing therefore turning it has no effect.
It seems like you have not done anything to negatively impact the system but the fact still remains that if it is indeed a heating system the pressure may be set too high (depending on local codes and regulations). If it's a PRV for your domestic system (tap water) then 2 BAR is most likely close to the correct number (also depending on local codes and regulations).