Yeah, that's perfectly legal. Just remember to only roll as many dice as you have of the first type of save before you move on to the other type of save: if you have 2 TH/SS termis out in front then 3 2LC Termis behind them, it's not kosher to roll all 5 AP2 hits you've taken at once, but rather, roll 2 (remove any casualty SS Termis), then roll as many dice as you have SS Termis remaining, etc, until you've killed off enough minis to get a different save.
A canny opponent will work around that, of course. There isn't a lot of AP2 Barrage weapons in the game, but barrage do deal casualties from the template's center, not from the front. A fast opponent (say Eldar with their mobility and semi-Rending shuriken weapons) will zip around behind or to the side of the squad to shoot at the more vulnerable folks first. Or, in the case of that Termi squad, they might bombard you with non-AP2 weapons until you roll a few 1s, so that you're taking everything on 2+ regardless, and then once your SS-bearers are down, they'll let rip with the AP2 stuff against the squishier other Termis.
So it's not an impervious tactic, but it is a good one!
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!