Coming from the other end as an inexperienced Dom, and who would be more likely to be overly sensitive than insensitive ("OMG didn't that HURT?"), I'd definitely want both Dom and sub helping to train me to be experienced, and experienced with each other.

The Dom and sub would each have to know each other's current limits. And if I were the Dom being trained, I'd want to know that was the case. That way I'd feel free to try what I thought was right, secure in the knowledge that one or both of my mentors would push me further when necessary, stop me if I went too far.