If you have the 2% and you want a 5% Equivalent use 2 1/2 times the amount.
If you have a 16 oz of the 2% let it evaporate by 50% + some and you will have the 5%
(actually the official range is 4.5-5.5%)
In any case the discussion about increased potency if diluted material is used is worth noting. Homeopathic remedies are based on such a principle.