If you measure between the bottom (or top - doesn't really matter) of the bed and the rail that it rides on, at front/back/right/left, are they all very similar measurements?
If not, fix that first. Reasoning through the leveling processes here:. Manual level: M1000 (I think) puts the nozzle at the Z-offset position, so that you can physically adjust the gap between the bed and the nozzle without any software interference. Auto Level: uses the process th develop a digital "mesh" of the bed warp, and while printing, adjusts the Z-input values along with the X and Y to position the nozzle where it needs to be. If, at X200Y200, your bed is 3mm lower than your print nozzle, and at X100Y100, that gap is only 2.5mm, the firmware on the printer remembers that and adjusts the nozzle as needed to keep the expected "layer height".
So, my best practice process:. heat up the bed (you'll want it to be at temp to warp as it will). run the manual level print job (or enter M1000). do the physical adjustments at the 4 adjustment points. then (if you want it really good):. do the auto adjust process, followed by printing the test print and adjusting the Z-offset as needed. Now, the last time I did the auto level, while printing the test print, in the left front corner of my bed, I noticed that the nozzle was lifting as it printed in that area, so I checked the mesh map and it showed one point much higher than the rest, which wasn't really true, so I adjusted that specific point to be in line with the rest of them.
It's not an easy process and I get frustrated every time I go through it, but for such a large print area, it's a necessary evil.
For dual extrusion it would be a bigger modification i think.Do you want to go for a second extuder and dual material with one nozzle or dual nozzle or do you want a second gantry to print parts parallel?