How many liters of hydrogen per minute are you producing with your second unit? And will it's production meet at a minimum of 5% of the total volume of diesel fuel/air ratio per minute that your engine requires to operate normally? That is usually the problem. Then metering the amount in a means per cylinder. Injection timing is crucial with diesel. If you made 5% to 7% per the necessary volume then lessening the amount of diesel injected per that volume would be needed to achieve the gain.
Again this guy spells/mathematically it out for you in regard to volume @ rate, etc. Hadn't forgot about some references for you, I've been on jobs for a while.
http://www.youtube.com/user/Silverad.../0/oP6woNV4IvU