Using the developer consumes it pretty significantly, and leaves behind products of development and things from the film. So you can't re-use a developer that was not designed to be re-used, sorry. Your second roll is likely to come out significantly thinner (underdeveloped).

As to your "more what?" question: increasing any one of concentration, temperature or time will increase the amount of development action to your film, which in turn increases the contrast. Under-developed film has too-low contrast and (often) no shadow detail, whereas over-developed film has too-high contrast and will start to lose highlight details. If the contrast is too extreme in either direction, the negative will not be able to be printed cleanly on photographic paper; it might be rescuable via scanning but if the contrast is so whacked that it doesn't print well, it's likely to have faults in addition to extreme contrast that are much harder or impossible to correct. A bit of contrast variation is no problem and there are frameworks (see below) for deliberately varying contrast during development in order to optimise your images for the contrast in the scene.

The definition of "too much" or "too little" contrast depends on your paper. Modern variable contrast papers will successfully print a wide range of contrasts, which is nice because we get significant variation in contrast from the scene lighting and from differences (or mistakes) in development technique. If you want to know more, google up "Zone System" and "BTZS" and buy a copy of Way Beyond Monochrome; it is an excellent reference on B&W technique. There's a FAQ in my signature that I suspect might help you too.