the easiest way i can explain it is to think of it like plumbing. (this is doesn't really bear much similarity from what is actually happening but it's a good model to work out the numbers.)
you battery is the water supply.
the voltage is like the water pressure.
the motors are like pipes which allow a certain amount of water through. the amount of water going through is like the current.
the pipes/motors will only take a certain amount of water pressure (voltage) before they burst.
the pipes/motors will allow a certain amount of water/current through them at their rated pressure/voltage.
if you supply less pressure/voltage the water/current passing through them will drop.
if you supply more pressure/voltage the water/current passing through them will increase but at the risk of damage.
now, if you connect 2 supplies/batteries in series you get twice the pressure/voltage.
if you connect 2 supplies/batteries in parallel you get the same pressure/voltage but they contain twice as much so you can draw water/current for twice as long.
if you connect 2 supplies/batteries in parallel you must make sure they are at exactly the same pressure/voltage otherwise water/current will flow from one supply/battery to the other. (make sure both your batteries are at the same state of charge.)
if you connect 2 pipes/motors in parallel to a supply/battery each will still get the same pressure/voltage so each will continue to draw the same water/current.
now my analogy starts to break down a bit here...
if you connect 2 pipes/motors in series to a supply/battery the pressure/voltage will be split between the 2. each pipe/motor will only get half of the pressure/voltage so will allow less water/current through.
so, applying this to your situation,
I'm using a 12V 20 amp hour battery in my robot. I know that the battery is able to last for twenty hours, if its constantly providing only one amp of current throughou the cycle. So for instance, if I used two 2amp motors, in my robot, not counting any other components, the battery would last for about 5 hours, right? But if I used two 1 amp motors in my project, the battery would last about 10 hours? Do I have it right?
yup. right on.
Another thing, I know that if you wire batteries in series it increases amperage, right? (Or was is parallel. Have to read up on it again.) So if I wired 2 of the 12V 20amp hour batteries to increase current, does that mean the batteries will push twice as much power through the circuit, or does that mean I'll double the life span of the power supply cycle?
if you wire them in parallel you would get the same voltage out of them and double the life expectancy.
if you wire them in series you would get the double the voltage.
if you are connecting multiple battery packs you have to be carefull to make sure they are the same specification, same level of charge etc.
imagine connecting a fully charged battery in series with an empty battery.
the empty battery's voltage has dropped off so it is not supplying the circuit with much current any more but the fully charged battery is still forcing current through it.
this situation is very bad for the battery.
if you connect a fully charged battery in parallel with an empty battery the fully charged battery will try to recharge the empty one.
this is not too bad for rechargeable batteries (as long as the difference in voltages is not too high so it doesn't happen too quickly) but would be disastrous for non rechargeable batteries....
i hope this makes things clearer. if not, please ignore all of it. it worked for me when i was trying to figure all this out.
it also bears no real resemblance to how the system actually works so don't expect to be able to pass any physics exams with it....