It's not really a question of what teams can and cannot. It's more a question of how much HP does your tank have. When you're solo tanking it, the challenge hits a shadow for roughly 1050 damage when it ticks, and it ticks once per second. It's internal damage, so that means the pre-mitigation hit is 1364. That's not too much damage, but you have to sustain that for 20 seconds. Thus, 27280 damage pre-mitigation overall. So, Thiol is losing 22097 HP for each tank challenge. That's not that bad if the other tank is taking the boss back (though I will say that your Jugg tank would only lose 20733 HP, and that's if he never uses Force Scream), but it's still a lot of damage. More importantly, it's completely avoidable damage.
The only advantage I see to solo tanking the challenge is it makes the tanking a little bit easier in the first few seconds after the challenge phase. But as long as your tanks complete their challenge first (something that is easily ensured by coordinating start times), they will be out and controlling the boss well before it becomes a problem. Dual tanking leaves more healing margins (i.e. if a tank gets spiked right before the challenges, there's no need to waste mana that could be spent on the healing challenge) and is good practice for Nightmare Mode, since I'm quite certain it wouldn't be possible to solo tank it there.
You are talking like the tanks don't take any damage when they go in together. They do and sometimes when one tank is low for whatever reason, they may not survive in there. 20-22k damage isn't alot when your tanks have 40k+ HP and they can be quickly topped off.
Saying that you can't or you never should one tank the challenge simply isn't true. Being flexible is good. If for whatever reason you can't put 2 tanks into the challenge, then 1 tank can do it. I think one of our tanks even died in this week's farming run and we were forced to solo tank it.
Speaking of Nightmare mode, which we don't know the mechanics of. What if the enrage timer is tight? Why waste time to coordinate and wait? This is assuming that one tank can even survive a tank challenge on NiM mode.