Thank you for the additional information.
I am glad to hear that you only have two affected and I would consider keeping them separate (ie. not even sharing a gate between) from any other sheep you have on site. The presence of wood in the wool on his neck and the lack of wool on the back of the other are very suspicious that they are removing the wool via scratching and are itchy.
Itchiness and associated patchy wool loss in sheep can be caused by a range of mites (which are actually noticeable in a lot of countries). The mites we tend to have issues with are Sarcoptes scabei (which are contagious to people), Chorioptes mites, and Psorptes ovis.
While the mites don't always do what we expect them to do, Sarcoptes tends to be seen causing signs on face and ears of sheep and Chrioptes tends to be seen in the region of the limbs. But the mite I am particularly worried about (Psorptes ovis) tends to start on the shoulders, back or the tail area since they prefer areas that are well covered by hair.
As they set up their infestation, they tend to cause a thick yellow material to form on the areas that are affected (this is from the lymphatic material the mites actually suck from the skin.) This stimulates an immune reaction by the sheep's immune system. The area will swell and serous fluid will seep to the surface creating a crust and scabs. This will then cause the wool to fall out of the sheep because they either rub it off or pull it out when biting at the very itchy lesions.
Since only two are affected and it hasn't progressed to severe wool or skin trauma, weight loss, anorexia, and severe itchiness, I would advise having the vet have a look at these two. The vet will need to perform skin scrapings to identify this mite which tends to hide at the edges of the crusty lesions. Once identified, the vet will be able to advise you if you are in a country where reporting this disease is required and will be able to advise you of treatments available for your wee sheep.
I hope this information is helpful.
Please let me know if you have any further questions.