I can't really see why it would be dangerous, depending on what a recreational dose of tianeptine is to you. Obligatory disclaimer, I'm not a doctor, pharmacologist or medical professionals of any kind but I can't really see that these drugs have very much overlapping action.
I did a little googling out of interest before I posted in case I was overlooking something major and I found a reddit thread of someone wondering how safe this was therapeutically, some people were saying they combined them and they weren't really synergistic but almost cancelled each other out. I did find a report of a woman who ingested a massive amount of both in, possibly, a suicide attempt, like 400mg+ tianeptine, can't remember how much amitriptyline but a lot - and was successfully treated with naloxone. She had no history of opiate use and the report just reads like a fairly uncomplicated opioid overdose to me. This was from 2010 - which IIRC is maybe before tianeptine's opioid activity was widely known, which would explain why the paper is written as if they're kinda confused about what's happening.
I've seen tianeptine described as an "atypical tricyclic" and I don't know how accurate this is, and I guess would give you pause when considering mixing with another TCA, but it's certainly very atypical and does not have very many problematic interactions AFAIK and IME. It would be more accurate to consider it an atypical opioid, IMO, since the opioid activity in actual fact seems to be the most important aspect of it's activity profile with a little AMPA and BDNF weirdness mixed in (IIRC, again)... which shouldn't be much to worry about.
I'd say if you start cautiously, maybe in the 40mg range for sulphate or 25-30mg for sodium/Stablon... for me this would be just about recreational and I'd hazard a guess that twice that would probably still be in the range of relative safety... obviously you may want to adjust that depending on bodyweight, innate tolerance, and other factors... tianeptine is short acting enough though that I'd say if you start cautiously you should know within an hour or 2 at most if anything concerning is going on, and you can decide how to proceed from there...
Again though, not a doctor! I admit also not really knowing too much about tricyclics in general, so take my input with the appropriately sized grains of salt.