I wanted dual Z screws with single motor, and Supasorn's Tarantula solution was very simple and elegant. I remixed this to fit Anet A2. The difference is that on A2 the distance between vertical frame and X bar is bigger, so motor mount and top mount had to be edited. I moved the position only 1mm further out on both, that is enough. I also edited the switch mount to match the switches used on A2 and moved the switch position on motor mount accordingly. I have added a new holder for X endstop switch, as the original doesn't fit in the correct position any more due to the 2nd leadscrew.
Supasorn's original lead screw nut holders should be used, I have included the files now for convenience, they have not been changed from original.
All parts use M4 screws & T-nuts, except motor mount, which uses M3 screws/nuts. All could easily be edited to use other sizes, but I left them as they were, as I had all sizes available. If you want to use Anet original M5 screws/t-nuts, just print the parts with 100% infill and drill out the mounting holes with 5.0 or better yet, 5.5mm (to give you some alignment room) drill.
Edit: full screw list, as it was requested:
10pcs of 412 CS + 4mm washers (2+2 for each upper mount to frame, 2+2 for nut mounts to X-bar, 2 for X-switch mount)
8pcs of M312 BH + 3mm washers + locknuts for upper bearing mounts (2 per side) and nut to nut holder (2 per side)
6pcs of M38 BH for fixing motor to motor holder (4) and Z switch riser (2)
1pcs 320 screw for Z height setting (I could get away with 10mm, but YMMV here, as I'm running a custom bed mount and glass on top of the bed)
2pcs of M310 (M38 would also do) to fix motor mount to the frame
Original screws for X and Z switch mounts (28 or 210 CS, if lost/stolen/broken)
Original grubscrews for pulleys and upper bearings
It's perfectly usable also as single inverted Z with original leadscrew - just print the motor mount, Z switch mount and right nutholder.
To use this, your Z motor direction has to be reversed from stock! You can do this either by using Marlin firmware and setting
#define INVERT_Z_DIR true in "configuration.h" or very simply by switching either black and green OR blue and red wires in the Z stepper cable, this reverses the motor.
I'm very happy with how it works, adding 2 pics of my latest taller prints (~100mm), absolutely no Z issues.
UPDATE: added right top mount that includes mount for A2 stock extruder
NB! Read also the "Post printing" part!
All pieces can be printed without supports, if aligned properly (top mounts must be rotated 180 degrees). I used PETG, but I suppose any material will do, although I have a feeling that PLA motor mount might be a bit brittle and break, when X bar is lifted by hand with serious force. If using PLA, I'd print 2 motor mounts and keep the other one as a spare. Use high infill - 70 or 100%. I avoid values between 70 and 100, because I've seen a test where those were actually weaker than 70%.
As with all Z setups, the most critical part is lead screw alignment. First mount the motor loosely to the motor mount and the assembly to the frame, so that motor rests on the table (your printer IS screwed down to a table, I hope?). Second thing is to align the nut holder on the X bar so that the screw is in precisely vertical position (in both X and Y direction, and in X direction it should be on the centerline of the motor mount. The nut should be resting against the bottom of X bar. Oversized mounting holes give you some room to adjust. Finally adjust the motor in Y dimension so that the lead screw is the same distance from the frame in the top and in the bottom, when the X bar is at the top. Check for smooth movement - if issues found, repeat the adjustments until it moves smoothly up & down.
For the second lead screw you need to start with the top, because that's how you tighten the belt. Fix the top so that the belt is reasonably tight (no need to go overboard), mark the position on the top frame. Fix the nut holder to the X bar so that the lead screw is vertical in X direction, and adjust top bearing so that it's parallel to frame in Y direction.
To align the top and the nut holder perfectly I then removed the long leadscrew and used the original shorter one - lower the X-bar and screw it to the nut holder so that it's a bit lower than the top bearing. Loosen the nut mount screws and wiggle around until the top of the leadscrew points exactly at the top bearing hole. Tighten screws and repeat for top mount - drop the leadscrew towards the nut through the bearing, but don't thread it in. Loosen top mount and wiggle around (within the markings, don't move right-left, as this would affect belt tension!) until it's pointing straight at the nut. Tighten. Alignment should be good now. Fit the longer 400mm lead screw (I'm using 500 because that's what I had and I didn't want to cut it, but 400 is enough), pulleys and belt. You should be able to move the X-bar up and down quite easily and without any "tight spots" by turning the lead screws.
Finally, to adjust the level of X bar, bring it close to the top, loosen one of the pulleys and adjust ONE of the lead screws until the distance between X bar and top frame is the same on both sides. Assuming your frame is square, this should be then horizontal. Now relevel your bed and enjoy.