The only time I've seen that done outside TNG is the Disco Volante, the villain Largo's ship in Thunderball. The front half of the boat is a hydrofoil. The silly gimmick was to allow the families of the TNG Enterprise to serve aboard a ship of the line. Splitting into two ships supposedly was for their safety, but I'm not sure if it was ever done but once or twice in the entire series run.
i always had a problem with this idea of the saucer detaching to allow the families to escape to safety in a combat i mean if an enemy attacks the saucer only has impulse engines (as far as i know anyway) to escape so it wouldn't get very far.
Thus is the battle section is crippled then the enemy can still attack the saucer.
Unless you know your going into battle then you could leave the saucer parked somewhere safe.
But this rises the question to my mind of why did the odyssey (i think that was the ship name) in ds9 drop off its non combatants at ds9 rather then just park the saucer?.
my inuniverse theroy is it was intended for that purpose but then starfleet thought it a bad idea and never carried over to many other ships, but left the option there for the galaxies.
but that i guess is a discussion for another thread