No, you don't need two layers of Hardibacker.
The purpose of the tile backer board (whatever it is; Hardibacker, Wonderboard, DenShield or even Greenboard) is to provide a dimensionally stable substrate for the tile that won't swell and shrink with changes in humidity and temperature like wood does. (And also won't disintegrate if it gets wet like drywall and greenboard do.)
So, if it were me, I would probably put up 1/2 inch DenShield (or even ordinary drywall) and then your 1/2 inch Hardibacker over the DenShield. The tile backer should not go behind or in front of the tub lip; it should stop 1/8 to 1/4 inch above the top of the tub lip. So, it's a good idea to go to any metal supply shop and get some 1/8 inch thick pieces of steel strapping about a foot long, put a board across your tub, put the 1/8 inch steel strapping across from the board to the tub lip, and set your heavy Hardiboard on the steel strapping to hold it a uniform 1/8 inch above the tub lip while you secure it in place.
If it were me, I'd just buy some stainless steel flat head screws of the length you need to dispose of the screw length problem. It might cost you $30 or so for 100 screws, but that's peanuts compared to what you're saving by doing the work yourself.
And, I would predrill the holes through the hardibacker with a 1/8 inch masonary bit, extend the holes into the studs with an X-tra long wood bit, and then counter-sink the holes into the Hardibacker with a 1/4 inch shank die grinder bit like the second one from the left:
Use a putty knife to schmear some thin set over the holes before you drive your #8 or #10 ss screws in, and the thin set will accomodate any difference in shape between the screw head and the countersink, and the excess thin set will get squished out of the hole and you can scrape it off with your putty knife.
Then paint over everything with a waterproof membrane like RedGard.
Then tile over the RedGard.
And if this is your first time tiling, you should know not to set your first row of tiles on the tub or shower pan because even though it might look straight and level, it might not be. So, if you set your first row of tiles on the tub, then all of your tiling is going to be as crooked as the tub was set.
What you want to do instead is nail or screw pieces of STRAIGHT wood molding to the wall around the shower area, and set your first row of tiles on that wood molding. Then, when all your tiles are set, you go back and remove the wood molding and cut your bottom course of tiles to fit down to the tub.
You want to choose the elevation you attach that board carefully. Lay out a row of tiles along the floor with spacers between them so that you can measure up from a hypothetical wood molding elevation to see what's going to happen at the ceiling. If it turns out that you're going to have to cut 1 inch tall tiles to fit to the ceiling, then you want to incorporate a horizontal border into your tiling that'll take up 1 1/2 or two inches of height so that you end up with almost full tiles along that top row too. And, you can choose various widths of border tiles to get the top row to be nearly full tiles.
(Similarily, you want to choose the starting position on that wood molding carefully too. Ideally, you'd like to be cutting almost full tiles to fit into the corners as well. However walls aren't aways straight and they aren't always vertical either, so you want to pick a starting position so that you end up cutting 1/2 inch to an inch off the tiles that fit into the corners as well. That way, if the walls aren't vertical, you don't end up with a 3/4 inch wide grout line in a corner because the wall and tiling diverged.)
Once you set the first 3 or 4 rows of tile on that wood molding, and allow the thin set to cure, then you can remove the wood molding as the tiles won't slip down the wall after the thin set has set up for a day. However, I always wait till the end and cut the bottom row of tiles last.
Typically, you'd want to be able to cut 1/2 inch off the bottom course of tiles so that that bottom course looks like full tiles but still allows a full half inch of wiggle room so that if the tub is 1/2 inch higher at one end than the other, you can still get the tiles to fit. The flatter and more level your tub and the straighter and more vertical your walls, the less wiggle room you need. Still it's best to lay some tiles out on the floor both end-to-end and side-to-side to see how things will work out with each hypothetical starting point on the elevation of the wood molding you choose.
I always use 6 X 8 inch tiles because that's the largest size you can comfortably handle with one hand. That's because the way I set tiles is different than most. Most people spread the mastic or thin set on the wall and then set the tiles with both hands. I spread thin set on the wall first, and then back butter each tile with wet thin set. That way, if the thin set on the wall dries and skins over, the moisture from the fresh thin set on the back of the tile will re-activate it so that I don't have an issue with the thin set not wetting and sticking to the tiles. And after 21 bathrooms after 25 years, I have yet to have a loose tile.
The reason why you might have that extra layer of drywall might be because you have an older house that originally had plaster walls. It may be that a previous owner took down the plaster and put up drywall. But, since plaster is typically 3/4 inch thick, he should have put up two layers of 3/8 inch drywall instead of two layers of 1/2 inch drywall. It could also be that the walls aren't straight and there's a transition from plaster to drywall somewhere in your bathroom. The last person to do any renovations in the bathroom may simply have noticed that the plaster was particularily thick and realized he needed two layers of 1/2 inch drywall for the drywall to be flush with the plaster. Plaster was all applied by hand, so it's not uncommon for plaster walls to vary in thickness by quite a bit. You can go from 5/8 inch thick to 1 1/4 inch, and everything in between.
Post again if you have any other questions.
(PS: The first thing you should do is use some masking tape to cover over the rubber stopper in your sink drain so that it doesn't accidentally pop out and you get a bunch of crap fall into your tub drain. Also, put a piece of scrap carpet in your tub pile side down to protect the finish of the tub from being scratched up during the work. You wanna use a thick carpet that won't grind any pieces of tile into the tub enamel.