You mention "unless you intend to face tank the soul reaper DOT too"
are you saying the tank would only need an external cooldown like hand of sacrifice to survive the second hit? provided the tank still has barkskin up?
Just would like more info on how you would reccomend rotating cd's properly?
Yes, that is certainly possible. Though it might not always be advisable.
Let's say you're a Bear tank, and on average, you take 43k in melee hits without CDs (this 43k is already after Demo Roar, Inspiration, etc), You're also going to suffer the 70k DoT along with one of these hits. We are going to assume you have the 4p bonus up, and that you have a Paladin's Hand of Sacrifice.
After various DR effects, the 70k is mitigated to: 70k x 0.97 (Blessing of Sanctuary) x 0.88 (Protector of the Pack) x 0.88 (4p bonus) x 0.7 (Hand of Sacrifice) = 36.8k
After Hand of Sac and Enrage 4p bonus, the melee hit is mitigated to: 43k x 0.88 x 0.7 = 26.5k
These add up to 26.5k + 36.8k = 63.288. A BiS Bear would certainly live through face-tanking a DoT + melee combo, although your healers are going to need to top you up ASAP afterwards because the next melee hit comes 0.9 seconds later (LK gives himself a Haste buff after the DoT).
The other thing to keep in mind is Hand of Sac's health transfer limit. When given to you by anyone other than a Prot Paladin, you're going to likely lose the buff well before the full 12 seconds are over. Moreover, the 30% DR is on the low side, so it might be better to pair that with a slightly stronger CD like Barkskin as opposed to the 4p.
Here's an example of a strategy my guild found great success with for Reapers. We most commonly ran Bear MT Pally OT, or 2x Pally tanks. In the example I'll show what we did when it was Bear + Pally. Half of the time, the Bear face-tanked the Reapers with a small CD of his own, along with a big external CD from the Prot Pally. The other half of the time, the Paladin would tank.
0s 1st Reaper -> Bear 4p, Sindy trinket + OT taunting before the DoT ticks and face-tanking the hasted melee hits with Divine Protection and his 4p bonus.
30s 2nd Reaper -> Bear Barkskin + Divine Sacrifice, Divine Guardian from OT.
60s 3rd Reaper -> Bear 4p, Sindy trinket + OT taunting before the DoT ticks and face-tanking the hasted melee hits with Pally 4p bonus + glyphed Hand of Salv, and TOGC trinket.
90s 4th Reaper -> Bear Barkskin + Hand of Sacrifice from OT
120s 5th Reaper -> (Same as the 1st Reaper)
150s 6th Reaper -> (Same as 2nd Reaper)
One strength of Bears on this fight is they have several small cooldowns on a 60 sec CD that they can rotate on reapers. Since Reapers are 30 seconds apart, this means they can have 1-2 small cooldowns for each Reaper. We coupled 1 small CD (from the Bear) + 1 big CD (from the Prot Pally) to get him through most Reapers. The pally would taunt some of the time, as described above. This entire time, the Bear would keep his Survival Instincts and Enraged Regeneration in the tank, to be used in case of a contingency. If a Holy Pally was grabbed, he would pop Enraged Regen right then. Survival Instincts he would try and hold on and to use when really needed. Things like Pain Suppression and external CDs apart from the Prot Pally's were also employed mainly just as back-up tools, since the tanks were able to do such a good job keeping Soul Reaper under control between themselves. Divine Sacrifice was used just before the 2nd Reaper because then it would coincide with an Infest cast, thus the raid would benefit from the 20% DR effect of Divine Guardian too.
For the most part, the small CD + big CD combo worked out amazingly for face-tanking entire reapers, especially since Bears are already inherently such EHP behemoths. If there is the slightest chance your tank might die from the DoT tick + melee hit though, it's safer to just have the other tank taunt 1.5 secs before it ticks. That's the most foolproof way to do it. The advantage of allowing a tank to face-tank reapers through the use of cooldowns on him are that a) your melee lose less DPS from the movement, and [B)] your rsham, rdruid, and hpriest don't lose time switching their healing target to the new tank (this doesn't apply to Hpallies because they are always healing 2 tanks anyway).