Just for the record, I'm in favor of dual screws or dual r&p over single screw any day. The single versus dual drive systems both have their benefits and detractions. The moving knot suggestion is just a way to help minimize racking in the event that you go with single screw and have some racking issues.

Slaving X/A or Y/A is how I built my current two machines and I don't regret it at all. I did have problems with the first attempt but that is history now. It was an incorrect Mach3 related setting. Paying more for a second motor and screw are just what I would do without a thought about cost. To me it's just a necessary expense.