I am not sure which way would be best, I would think connecting two motors to a single drive would be problematic as you could only have one motors encoder feeding back, also you would be tuning the drive to the two motors rather than a single.
Having said that slaving a drive and motor to the master drive and motor may also cause problems lke you say.
I think what you would likely need to do is slave one drive/motor and do an auto tune on the master then enter the values that are in the master to the slave then tune again. Might have to do that a few times.
Auto tuning may only get you close, all depends on the drive, for example I feel the auto tune in my AB drives is not great and I usually manual fine tune after. The Telemecanique drives however have very good autotune and so far have not had the need to manuallty tune.
Regarding the wiring diags, it is the first one you would use, however you would not have the step/dir signals connected to the slave at this point, only would you do that if you went with Mach controlling it as a slave rather than having it slaved to the other drive.
Hood