Does premium set stack?

My current premium set is set to expire on the 22nd and I have 2 30 day sets in inventory. Can I use both right now to extend it 60 days or do I have to use one at a time?

You should be able to stack Premium; I have stacked a year's worth before.

However, other things can only stack up to 90 days, like Material Storage or Client Order expansion.

Awesome. Thanks!