I will answer your question following the text you have posted (in Italics) adding the answer in Bold text.
Moreover I assume that your 1S battery is a typical LiPo cell and that your 2S Battery has the typical wiring diagram as follows (the colors of the cables may differ), but the ones shown in the picture are the most common ones:
Answering your questions:
1. *can I use up the remaining charge (plugging the battery into a random
appliance) until that multimeter reads 3.8V for the 2s pack?*
You may discharge each cell connecting for instance a resistor, until each cell reaches 3.8V (Measuring the voltage between the blue and red wires for the first cell and between the black and the blue wires for the second cell), that means the 2S Pack will reach 3.8+3.8= 7.6 Volts (measuring the voltage between the red and black wire as shown in the picture). Do not try to discharge each cell below the minimum voltage shown in the battery data sheet -usually Never below 3V per cell that means a minimum of 6V for a 2S pack). If you try to discharge the cell further, be aware that the battery may get damaged and in extreme cases catch fire or explode
2. *When I check the voltage of each cell in the 2s pack, the first cell
reads a voltage of roughly 4 volts, and the second cell reads around
8 volts. Why are the cells having two different voltages?*
If you get those readings that means that probably actually you are measuring one cell reading about 4 V (for instance between the black and the blue wire) and in the second case you are measuring two cell S. (for instance between the black and the red wires reading about 8V). Measuring each cell you should be reading about 4V for each cell.
3. *What voltage should I store in the single cell?*
Usually 3.8V is a good figure, but please reefer to the data sheet of your battery.
Please be careful performing these measurements, treat, store and use the LiPo batteries properly, reading the product data sheet and acting according to the manufacturer reccomendations.
Enjoy and be safe.