I believe 2 hooked in sieries is the only way it will work very well.
Remember, DI units depend on resin contact time with the water. As your cartridge begins getting used up (color changes), its effectiveness goes down.
Running 2 does 2 things: first, increases contact time, and second, ensures that you can completely exhaust a cartridge before replacing it. If you only run one, you probably need to replace the cartridge when you can see there is only an inch or so of usable resin, for fear that contaminants can leak through.
When one is used up, take it off, 'promote' the other cartridge to first in line, and always add a new cartride so it is last in line. This way, water always passes through at least 1 *entire* fully charged unit.
good luck,
Steve