If you aren't "stacking" the staggered condition into the next higher condition (stunned) then like every other condition adding effect, you end up "stacking" the duration to last longer.
Both the Staggering and Stunning Critical feats expressly state the conditions do not stack, but rather add to the duration. A successful save against the stunning critical would be reduced to staggered and count as an additional hit with staggered critical for the sake of determining the duration of the staggered condition from your staggering critical, since both are active through Critical Mastery.
I don't know why you would want to stack them other than why not, you already have them both and can't retrain out of staggering critical as it is a prerequisite for stunning critical.
Should have went with Blinding Critical, and Bleeding Critical if you were planning on stacking conditions with Critical Mastery, in my opinion.
Unless you want stunning critical for use in conjunction with Dastardly Finish.