Just because it doesn't give MR to tanks doesn't mean it isn't giving them something they don't need. Tanks don't need tenacity at an efficient cost, and they don't necessarily want it attached to stasis. It's either going to be strong enough to just reduce the effectiveness of CC against them or worthless because it won't be defensively viable. There will be no inbetween so long as you also have to tune it for AP bruisers.
First, I can and will purchase both. It's not a Zhonya's or Spellblade, situation, it's a Zhonya's and Spellblade situation. Stasis is very strong. I have no reason not to purchase it twice. It's also not a waste of stats, and if it is, it won't be purchased because merc treads are an option and they waste fewer stats.
Second, even for situations where someone would only buy one, why is a choice needed? It just serves to increase the number of characters with access to stasis, and that's not something we need. AP bruisers already have their option. Giving them a different one that also has tenacity attached isn't necessary, productive, or useful.
And finally, why the heck should we have an item that is just a cheaper version of another item with the exact same main effect and one extra stat? What's the purpose of that, particularly when it's tuned to still be purchased mainly by a class that already can buy the first item?
I'm not swayed. It's a bad idea. It tries to fix a problem that isn't a problem, and it just overlaps with Zhonya's in design.