You can’t open a 40k blog these days without reading about how Look Out Sir! allows you to allocate wounds amongst multi-wound models in the same unit. But really, according to how I read things, you don’t need a character of any kind to do this, you just need an opponent who obliges you with units with different initiatives!
This will be short and sweet, with just a brief description of the rules I’m discussing, and a brief example.
The rules for allocating wounds in melee say that the player who is taking the wounds picks the model closest to a model attacking at this initiative step, and puts the first wound from the wound-pool on him. He makes his save, if he fails the wound is applied. This is repeated until the wound pool is emptied.
Now here’s the key part, the wound pool is emptied at each initiative step.
So at the next initiative step, you can pick a completely different model to put that first wound on, there’s no requirement to place the same wound on an already wounded model once the wound pool has been emptied from the previous initiative step.
Also, as an aside, note that there is no requirement to place the first wound on an already wounded model who suffered his wounds before the melee he’s now in, for example from Overwatch. Spread those wounds around!
Here’s an example: A unit of 6 Screamers of Tzeentch (Daemons with two wounds each, and Eternal Warrior) are fighting a unit of Assault Terminators (A few Thunderhammers, a few Lightning Claws) led by a Marine Captain.
The first initiative step (ignoring Hammer of Wrath) would be the Captain at I5. He has 3 attacks, 2 hits, 2 wounds, and the Screamers save one, so a model in Base to Base with the Captain takes a wound.
The next step is I4, where the Screamers go…let’s say they roll like I do and kill only 1 TH terminator. The LC terminators attack now, with their 6 attacks they manage 4 hits, do 3 wounds, and we save one of them…we have to place the first of these wounds on a model in Base to Base with a Lightning Claw (or the closest model to an engaged Lightning Claw...not necessarily one that's already wounded!), then have to place the second wound on the same model until the Wound Pool at Initiative 4 is empty.
Now the TH go, with 4 attacks, 3 hits, 2 wounds, we save one, and put it on yet another Screamer...one that's in BTB or Closest to a TH Terminator...not necessarily one that's already wounded!
Now we’ve suffered 4 wounds, which might have killed 2 models if you just put them on already wounded meanies, but instead we’ve only lost one, preserving 3 sweet sweet S5 AP2 attacks for next combat round!
You can apply this in many cases…Wraiths, Nid Warriors, Grotesques, Carnifex, etc. It gets a little weirder with Instant Death, as your opponent can force you to apply the ID wounds before or after the non-ID wounds at that step, but things can still work in your favor if you remember that once the wound pool is empty, you can start allocating again however you want, as long as the first model you put a wound on is in base to base or closest to an enemy attacking at this initiative step.
Now go forth and allocate wounds like a pro! :)