I am with Houie for sure but must add that underfeeding a pet for your purposes of convenience is not an acceptable practice in my book.
I am surprised at the slow growth of your TSN. Not that it does not happen. It's rare. Was not your TSN kinda strange in color and in the absence of growth past 1.5'? I am much more surprised at your TSN's absence of predatory practice. That ~ never happens (with healthy TSN).
In any case, IME TSN will grow to massive 2.5', 10-15 lb in ~3 years, while jardini will still be ~1.5' and slender, ~2 lb. But I think it would be eaten long before that (as a rule, not that there are no exceptions).
My 2.5' TSN was served a $300 dinner plate in the form of 1.5' Jardini aro by stupid me once. At ~3' he could put away 2' silver aros. Both types of aros were new comers in a 4000 gal 40'x6'x3' pond. If raised together or if the order of intro switched, things could be different but I'd imagine sooner or later TSN will eat up the aro and will go "You pleased my palate, Master! Got anything else that good?"