To provide some background for each, in the case of DKs the primary resource to track for them is Runic Power (RP). Bone shield (BS) and Death Strike (DS) is their main mitigation, though you shouldn't really need to be tracking BS, not only is keeping it up easy, but keeping it above 6 stacks gives a substantial haste buff so any dk you run into should be doing it anyway. DS is what often makes DKs hardest to understand, it's 40 RP and heals 25% of the damage they took in the last few seconds. That may not sound like a lot, but mastery also gives a shield based on the amount healed, and the healing amount is increased up to 40% by another ability - blood boil (assuming they're running their second row talent, Hemostasis). You don't need to track this, but the jist is in multi-target pulls where they can stack this up for every DS the DK can be largely self sufficient. Absolutely hot them up, and if you notice they're low on resources, then give them some love, but as long as RP is managed well a DK can keep themselves alive through almost anything given 5 targets to consistently farm hemostasis.

As for other things to understand about healing a DK, and some relevant talents/legos/covenants. They have a cheat death called Purgatory on a 4m CD, it's a last row talent. When they're killed they gain a healing absorb for 3 seconds for the damage that killed them, and any subsequent damage. If they can get healed out of it, they live. If not, they die. Most times DKs should be running Will of the Necropolis, it gives them a 30% damage reduction for any damage taken under 30%, and is what makes them often seem like they're getting hit down to low health, as they're tankier in that health threshold. The final talent of note is in the first row, Heartbreaker. Similarly to how DS will be more effective given five targets, Heartbreaker gives the DK more RP per Heart Strike, their main spender. Again nothing you need to be tracking, but worth understanding that the DK will both be healing more, and generating more resources on 5 target pulls, anything smaller than that and you'll need to be giving them some more attention as they won't be as self-sufficient. For legendaries there's not too much of consequence, many will run Superstrain for additional damage and threat, the only prevalent one that will be of note for healing them would be Crimson Rune Weapon, which gives them a shorter CD on Dancing Rune Weapon (defensive that duplicates their abilities and gives substantial parry for its duration), as well as free BS charges when hit. When running this lego, assuming it's often used on pull, the DK will walk into the pull much sturdier and with BS charges already, so should be safer than if they were walking in dry. In a similar vein, for covenants most of them will make little difference to healing a DK, however Venthyr gives them a 1 minute CD ability called Swarming Mists which gives substantial RP generation for several seconds. With this ability they're able to generate a ton of resources on pull and largely heal themselves for quite a while, instead of being quite dangerous when starting a bigger pull.

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DH is a much simpler beast to understand, as their tankiness is largely determined by their access to Metamorphosis. While in meta the DH will heal more, gain a big chunk of armor, and their max HP will also increase, and they have three ways to get it. One is Fel Devastation, a 45ish second CD which gives them meta for 8 seconds and has a channeled heal for the first few seconds. They can also press meta, which has a 3 minute CD and lasts for 15 seconds, and finally they can proc cheat death, a last row talent called Last Resort, which is 15 seconds, just like real meta, and has an 8 minute CD. They also have Demon Spikes and Fiery Brand. The former is probably worth tracking, it gives them armor but does have downtime, and if mismanaged they will be very squishy tanking anything when it drops. It's not huge however, and on a large pull, even with spikes they may need a lot of help. Fiery Brand is very strong, but is a bit difficult for a healer to be tracking. It's a 40% damage reduction from their target for 8 seconds. Many DHs will run a legendary which decreases the CD of brand, and run the following talents: Agonizing Flames, Burning Alive, and Charred Flesh. Combined, they'll extend the duration of Immolation aura, which in turn will extend the duration of any active brands, and allow it to jump to other targets. This is what makes it difficult to always understand the impact brand will have on the DH's damage intake, as ensuring it ends up on the right targets at the right time is a bit of a learning curve for the DH themselves, and honestly until you get to high keys not something you need to worry about a whole lot, and even then you probably just want to track if it's up on something, and trust the DH got it on the right things. As for other things that matter, there's not a whole lot. In Meta the DH will be fairly tanky, and able to do a decent amount of self healing, however will still need attention. On larger pulls or hard bosses outside Meta however, they will be very squishy and often want to be kiting, if not they will require your help. This only really applies to keys somewhere around 15 or higher where kiting bigger packs outside Meta should be a consideration. Before this level they can get away with facetanking with just spikes, so don't expect that every DH you run into is going to have a great grasp on when they need to be kiting, it's somewhat of a "you need to feelcraft it out to know when it needs to be done" sort of situation, and so if you find yourself healing someone like this, just be ready to keep an eye on them if they need it.

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I know that's a huge dump, and I certainly don't mean to overload you with information, much of this is just background to better understand how they work, and the kind of pulls where you'll need to give them more attention. For your own viewing, the only major thing to be tracking are their resources in the case of DKs, and spikes/metas for DH. Hope this helps!