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Thank You for feedback. Honestly I am a bit clueless, but I will try my best.
Add 1, I am aware of the fact, that for each and every stepper motor you should have one dedicated stepper driver. BUT ... one driver for two motors is working for NEMA17 and also with hacked 28BYJ-48 on other set-ups. So this should not be the case especially when both motors together need few mA only. Although you won't loose much trying Marlin with dual Z drivers.
Add 2, I use hot melt glue to attach M5 nuts to their position.
Add 3, Already sent source models to few people who wanted to do NEMA17 remix, but so far no one published the result.
I have few more questions for you
1, What drivers are you using? What is the value of sensing resistor on it? What is the reference voltage set for your motors?
2, Do both Z motors spin when not attached to the threaded rods? Is their movement fluent without chopping?
3, How are Z motors connected to RAMPS as there is not enough space to use the original connectors?
4, What values you have set in firmware concerning Z movement - max. speed, acceleration, etc.?