Hey Paul!
I ripped the internals out of my Rekord60 hood and bolted in two 15w T8's, more than enough for my 15-gallon.
Adding a second tube whilst keeping the original light doesn't work well in terms of light distribution - you end up with loads of light below the tubes but depending upon where you put the second tube, you end up with a dark patch at the front or back of the tank.
It's a little more money and a little more time, but if you strip everything out of the original hood you can bolt one tube onto the front vertical panel of the hood and one to the rear vertical panel of the hood. With the reflectors positioned correctly this gives a really even light spread over the whole substrate base.
I'll try to get a photo tonight when the hood is off for the water change.