They can both use roughly the same amount of nutrients. EC 1.6 with a PK-Booster like PK 13/14
I would feed them both with the same nutrient solution till your more matured plant (she is looking gorgeous) reaching time to flush. Then I would disconnect that one from the automatic system and flush her manually.
Ok ... I guess I'm not understanding the question... I thought you wanted to give one plant a nutrient that you didn't want to give to the other??? ... you want to add some PK to the feed? Why wouldn't you just add it to your reservoir? Doesn't your reservoir feed/water all the plants? What am I missing?!
If you're watering from the same reservoir, you can't... but depending on the nutrient one needs and not the other (which you didn't specify), there might be something you could top dress that would take care of the problem.
Good luck!