How to tune a dual drive AC servo system

Hello all,
I'm currently reading the forum and absorbing knowledge, before I start to think about designing an 8x4 router system.
It seems to me that the generally accepted way of building an 8x4 router is to have a dual drive system to move the gantry.
If I were to use AC servos in a dual drive configuration (e.g. using ballscrews) how would I tune the two servo drives? It appears as though almost all AC servo drives offer an auto tune function, but that only works for one motor at a time, surely you'd need to tune both servo drives at the same time with the gantry attached? How do you do that?

Many thanks
Bob