Good idea.
I have seen high end tanks that run propagation systems off of water changes from their main tank. And those setups are long term.. not just for new tanks.
Some even go to the length of having contiuos changes with a Litremeter 3. Fresh saltwater goes into the main tank, water-change water from the main tank is transferred into the second tank, and waste water from the second tank goes into the sewer, continuosly.
Either way, this method is dependant on having good water quality in the main tank to begin with and being very consistant with your changes...