If you are not taking out the viscous fan, then I can not see the point of putting in a second electric fan. It's just something else in front of the radiator to slow the air flow down.
The only reasons to fit an electric fan are
you are in vehicle with a light weight engine and need a few extra BHP that the fixed fan nicks... Not the case in a Terrano
You don't like the gratifying roar of the fan, and want to sound like a washing machine when the car is hot instead.
You tow, and turning the electric fan on a bit earlier helps stop the engine getting hot, but since the Terrano seems more than capable of coping with what ever you stick on the back, no need.
You spend a lot of time parked up in queues on the M25, not moving, but again, the Terrano seems to have no problem cooling it's self
Your Viscus fan has died, and you already have an electric fan knocking around in the garage, so it is easier and cheaper to fit that, than go looking for a replacement viscus one.
Remember when hardly moving the alternator is not putting out the maximum power, so it is easy to run the battery down when using it at slow speeds, especially if you have a second fan as well. Then you turn on the internal fans, maybe your lights, and try to open the windows, but the battery is flat. Engines that run too cool actually use more fuel as well.
So in a nut shell.. seems like a bad idea...:lol