1 year old cat eating kittens food

I have a 1 year old female (Prim), is recently at her "ideal weight" (could stand to lose a bit more, but I don't want her to gain) & I have an 11 week old male kitten (Pippin). They both eat wet & dry food.. but Prim keeps eating his! I can monitor the wet food but I'm not always there with the dry food.

Prim is eating Orijen Cat & Kitten, Pippin is eating Purina Pro Plan Kitten (only because I got a free bag).. I'm thinking of just switching the kitten onto Orijen (he eats hers anyway), and just get rid of the Purina?

But... they both only need a certain amount per day, how does one keep track of that? Before I got the kitten (2 1/2 weeks ago) Prim only ate a very strict amount.

I realize ideally they should just be on wet food but any tips would be appreciated

You're absolutely right. Prim should not have the kitten food, especially since she's slightly overweight. The only effective way I know for doing that is by feeding in separate locations (or separate times in the same location where one cat is locked away from the feeding area). I wouldn't throw away the Purina kitten food if the kitten likes it. Cat food is expensive and if you got a free bag, make the most of it, IMO. 

Here are more tips on feeding in a multicat household - 

Must-Know Tips for Feeding Cats in a Multi-Cat Household

I wouldn't worry about feeding dry. You chose a quality brand and unless your cats have special health concerns, I think feeding dry is a perfectly legitimate choice. There are many claims but little proof that dry is bad for cats. Just make sure they get enough water. If you feel this may be an issue, this article may help - 

Tips to Increase Your Cat's Water Intake

I feed in separate rooms I give half hour to finish then let them mingle, they learn the other will finish if they don't finish in the half hour allotted. This is ok because the big boy finishes what small girl leaves. I do timed feeding only all wet. At meal time the boy runs in back bedroom, he is used to eating there now. If I did not do this he would intimidate her and eat half her food.

How do you know Prim is over weight?  I find i hard to believe a 1yo kitten is overweight.  i would mix the two dry foods together and not worry about it.