Lets not put too much thought into this, this is basic suspension. Both front and rear are sealed units. There is no compression adjustment for either end, no preload or rebound for the front. In other words, there is not much you can do.
The OP did not say how much he weighed or what he carries on the bike. IMO anything over a 185lb would benefit with respring, more so, if doing any type of bumpy terrain.
Hyperpro springs give a pretty detailed instructions. You can buy a "kit" that includes front and rear springs along with the oil.
The forks are cartridge type, meaning on compression, the oil is forced around washers held in a stack of them. Enough compression force bends the edges allowing more oil to pass. Changing viscosity forces the oil to bend the washer rather than go around the edges. It just won't flow fast enough other wise. It slows the compression of the spring.
It is hard to explain the sophistication of the valves when crimped into the tube. They are pretty budget/basic.
Springs store energy when compressed. Once the force compressing them is released, they spring back. That rebound has to be controlled also. Again it is the viscosity of the oil that determines that, usually by being forced through a hole near the top of the cartridge tube.
Air gap is the volume of air left in the fork when compressed. Oil won't compress, but air does. More air will soften a ride, sometimes to the point of allowing the forks to bottom out, more oil will aid or prevent that.
The year I spent trying to turn the stock forks into something I could live with resulted in numerous oil viscosity's, air gap and preload adjustments(changing spacer height, even used spacer springs)
For my weight, terrain and riding style, I should have just followed the instructions. It was educational.
As for doing both springs? I recommend it. You will have different rated springs front vs back. They will compress and rebound at different rates.
I call it hobby horse effect, you feel it as two separate events as the bike travels over a bump.
When tuning suspension, it is easier to match valving with straight rated springs. Tuners hopefully match valving and most good suspension has controls to fine tune it. Variable rate springs complicate the tuners job, in trying to match valving need with whatever spring weight as it compresses. You don't have the ability to fine tune. I think I could do better experimenting with a straight rated spring.
For the average rider, the Hyperpro is pretty much plug and play and will cover a wide range of riders.