I have an ipad air 2. I renewed early (Dec) and it looked like at the time the countdown started at purchase according to the expiration date -therefore no stacking. My current 5gb plan wasn't due to expire until Feb. I believed I cut my 2nd data pass usage by 2 months.

But In March the newer 5 gb purchase expiration changed to July- So that means that It did stack behind my 1st pass and when my first pass expired, the 2nd pass date changed(from the initial expiration of May) to when it was first tapped for data in Feb when the 1st pass expired, but presently the 2nd pass still says pending. I read somewhere that the free 200 mb chunk will be used up first, then the data pass will be tapped and that pending status will change.
To test out binge on I watched 2 netflix shows 3 nites ago with wifi turned off. According to the t-mobile cellular data account accessed thru settings/cellular/manage t-mobile account, I still have 200 mb of free data remaining and my pass is still listed as pending. According to the mim.t-mobile website, it says the same. So maybe we get unlimited binge on due to the pass being > 2 gb??? Since the other nite, I installed a data watch program and will stream some more netflix and keep tabs on how much data the tablet uses. Then compare to what t-mo says I used.
I will post back with more details.