Simply Koi and More Koi

After readong on here that its gallonage through a uv that matters and not gallonage of the pond to the watts of the uv.
My turnover is 2000 gallons through a 15watt uv, I now have a spare 15watt uv to add to this makinkg 30watts (if thats right) should it be place in series or parallel ?

Regards

Russ.