they can't be stacked, and I don't think they can't be added to a previous sub either, unlike the CC ones lol. meaning if account ever had ps+, you're toast.
Not 100% on this, this is what I'm making from matsui's post.
Yeah, they can't be added to an account that at any point had plus, whether it be a trial or an actual subscription. They must have changed something recently (probably in the recent maintenance) as I remember using a trial code right after a previous trial expired (around new year) and nothing prevented me from doing so but if I try any trial codes now, regardless of their origin, I am unable to (yes, my plus has expired).