Xeno SM can be single-tanked. His big hit is Thermal Tolerance Evaluation - judge for yourself whether you need a defensive cooldown for it. You probably won't, in SM. The adds are a bigger problem, but just keep them off the healers and DPS - they don't hit terribly hard in SM, again.
In HM you need 2 tanks and you need to switch off at Thermal Tolerance Evaluation. Additionally, whichever tank that debuff is going on will probably need one (or two) defensives for it - on my Juggernaut I save Invincible *and* Endure Pain for it. I believe Sith Assassins can use Force Shroud to reduce the number of stacks they get from it. If you're the other tank, taunt during the cast so that as soon as it's over the boss is on you. Whoever has that debuff CANNOT take damage - it massively increases the amount of damage you take, to the point where a hit will one-shot you (at 9 stacks, anyway; unsure about lower amounts).
What my guild does is, after taking down the core and going back on the boss, the active tank (i.e., the one without the debuff) will move to position his back towards the door so as to more easily grab the adds when they come out. AOE taunt them, hold them as best as you can, and have the other tank single-taunt one off and start kiting him. DPS should burn down that single add, when he's dead have that tank single-taunt another, rinse, repeat. This isn't really necessary for the first group of adds, but the second group hits like a handful of trucks and if the DPS or healers pull off they *will* die. The offtank's debuff should be running out during the adds phase; once it's gone, he can stop kiting and actually tank the adds he's pulling. I usually save my other defensive (Saber Ward) for when I'm main tank on the group of adds.
With the right amount of DPS you shouldn't get more than 2 groups of adds before hitting the Final Evaluation burn phase. On this phase he'll do large amounts of damage via the Generate Urgency cast. His aggro switches pretty randomly; I'd recommend having the tanks taunt off as needed to try to keep him focused on them. A well-geared tank can take two or three (I forget which) Generate Urgency casts, from full HP, before the other tank will need to taunt. Make sure both tanks, then, and as many other raid members as possible are topped up before his shield drops and this phase starts, as once he starts casting the healers are going to be playing catch-up. If you have any defensives left, this is a great time for them, but only use them after a taunt when you know he'll be attacking you for at least two casts.
If one of the tanks misses a taunt (and thus the other tank dies), he *will* end up attacking the DPS or healers at some point because you're only going to be able to hold him while he's taunted. Just burn through, there's no need for high-aggro moves at this point and you should just be throwing as much DPS and defensive attacks as possible.
you don't need two tanks in HM ether it depends what kind of tank you have to for HM the boss any way .. true you need two tank but if MT is a Assassin and know his stuff no switching , I have done it on my war in HM with a Assassin as MT all I had to do was pick off a add on the second set of adds while he kitted them ..
I only died on the boss because I taunted and should not have ...