I know some people posted about holding 2 items and losing one of them due to overlap or something. Maybe that was the solution to the light source going out and needing to replace it.
Currently, if you are say .. in the Forest in the North Eastern section, you walk in with a torch... the torch goes out... it is pitch black and you can not light the torch that is in your immeadiate inventory.
Not sure if it is suppose to be this way, but even if you remove the light source from your grip to replace it with a new one, you end up with no light and thus are in a "pitch black" environment.
Every light source will warn you before it goes out. The light will "flicker" or "sputter" or do something else to let you know it's going to croak pretty soon.
You can use that knowledge to keep yourself illuminated. You're right to carry a spare light source with you into those areas. If you're deep in a dark place, hold the fresh light when the first one warns you. Then get out of there, and get new lights.
The alternative of course is to get a permanent, or long-lived light source. Again Poobah can tell you for sure, but I think there are quite a lot of them nowadays.
Would it not be possible to make it that you can access your spare light source so long as it is in your immediate inventory. I understand the idea that if it's pitch black you can't see to find it, but most people can find things with their hands and fingers if they can't see. You could make it random whether you get it immediately or not...like you say "get torch" it might come back with "you get apple" or something, but you should at least have access to your inventory in the dark. Shirrah had to do a number of pitch-black rescues for people who's lights went out and they were unable to get out their spares...