The double clutch and/or that push button thing on the gear shifter. Not sure if that’s part of the same system.
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The two parts you describe are separate things, but both related to the drivetrain of a vehicle.
Double clutching is a technique used to change gears. With some heavy duty or older gearboxes, the gears are not synchronised – what this means is that before you are able to select the next gear, the engine needs to be running at the right revs for that gear at the current road speed before it will slot in smoothly. Get the speed wrong and your gearbox will make some horrible grinding noises instead.
So what a driver will do is to use the clutch twice per shift. Once to take the vehicle out of gear and into neutral, where they can use the accelerator to increase or decrease the engines revs to suit the next gear, then use the clutch a second time to move the gearbox into the new gear.
This is one of those techniques that sounds a bit awkward at first, but quickly becomes second nature as you gain experience.
With a modern, manual gearbox car on the other hand, they use what is called a synchromesh gearbox. This is a gearbox that uses specially designed and shaped gears to allow you to shift into a new gear even if the engine is at the slightly wrong speed – you may feel a lurch as a mismatch in gearing and speed is forced to align together, but it works and allows for slightly less precision on the driver’s part.
The extra button or gearstick you see in trucks is gearbox related, but something different. At its simplest, it is basically an extra half step gear you can engage and disengage as needed.
With a normal petrol powered car, the engine will work reasonably efficiently over a fairly large range. With larger diesel powered commercial vehicles however, the rev range the engine will run efficiently over is much, much smaller. The result of this is that you need more gears to allow you to split the range of normal driving speeds over much narrower bands. The more gears in a gearbox, the more complex and expensive it gets, so trucks often use a two gearbox system.
One gearbox will be fairly normal, with around 5/6 gears. The second will only have two gears, but the jump between them will only be about half of a normal gear. By combining the two you double the amount of gears, so instead of changing between gears 1-2-3-4-5 like in a normal five speed car, they combine the two and use both gearboxes to change 1-1.5-2-2.5-3-3.5 and so on.
Exactly how necessary these will be will depend on the gearbox, the load and the road a truck is driving on – fully loaded they may need every single gear, while empty the truck might be light enough to allow them to skip some steps.
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