2 stopcocks under the sink for water supply? Cant find ext. stopcock?

Hi, I have moved into my first home. It is a mid-terrace Victorian property. I'm just trying to work out turning of the water supply incase of emergency. Under the sink there is 2 stopcocks. They both seem to feed into a shared pipe after the stopcocks towards the kitchen tap and beyond. To stop the water flow to the kitchen tap I have to close both stopcocks. If I individually turn them on, then one of them provides very low flow, while the other provides full flow. Can anyone explain why this is the case as from quick google searches it is not common? The "low flow" stopcock also has a very small drip from the underside of the stopcock. Is this easily fixable? Should I be concerned? I am also trying to locate my external stopcock. The surveyors homebuyers report says he could not find it, and it is not mentioned in any documentation from my solicitor. Who should i contact to locate this? Thanks.