the simple answer to your first question is, yes, by putting the 2:1 gear and then 4:1 gear, you essentially get a total reduction of 8:1 at the wheels, which will give you more torque to the rear wheels (up to a given rpm, more on that later).
for your 2nd question, which will also be an elaboration on the first question's answer, please understand that gears do not "give" or "take" torque, nor do they "give" or "take" power. I suggest you read up more on what gears and a gearbox does. It will be a simple read on wikipedia or something. It is not a complex subject.
gears basically shift your working rpm range of your motor by multiply your torque but at the same time dividing your rpm. a jet engine running at 100,000rpm with 10Nm or torque can still drive your wheels travelling at 500rpm (around 50km/h give or take) if it has a 200:1 gearbox reduction. This will then give you a torque of 2000 Nm. obviously the gearbox with a ratio of 200:1 isnt very practical, but it is theoretically possible. but the gearbox isn't giving or taking torque, it is just shifting the torque curve by the factor of the gearbox ratio. likewise, the power of the jet engine remains the same with or without the gearbox.
I'm sure others here will have a better explanation than mine, but for a start this can help you.
EDIT: oh yes, and to your 2nd question, it doesn't matter (in terms of power and torque) whether you use 1 or 2 gearboxes to reduce the ratio, for eg. a 8:1 gearbox vs a 2:1 then a 4:1. it does matter when you think of why you would want to increase the complexity of your overall car design by using 2 gearboxes instead of 1, or maybe the costs involved in making 2 gearboxes instead of 1 etc.