I don't think you quite understand how DTube works ;) As soon as you post on DTube, there is automatically an entry on Steemit as well since it both relies on the same blockchain (STEEM).
The only way he could do what you want is uploding it somewhere else like YT, then making an entry on Steemit linking there. But when he publishes on DTube a few hours later he will have a second entry on Steemit.
So it is neither practical nor can I see any sense in doing so :D