This question is fairly straightforward. However, how it is applied is often complicated.
If the PEB is able to separate the symptoms and disabilities and attribute them to two or more diagnoses, and they find each unfitting, then you should be rated on them separately. That part is pretty straightforward.
The application of this is more problematic. It is common for the PEBs to either attribute all symptoms to one predominant diagnosis, or to state something along the lines of "it is impossible to separate the cause of the each symptom." Neither is a correct way to adjudicate the case. In the first instance, what should happen is the evaluation (NARSUM) should attempt to make a differentiation (if at all possible), to state the reason for the findings, or if not possible, to state why. In the second case, it is simply not the fact that most cases are impossible to separate. They have to try, and if they cannot come to a decision, they need to state why. If they do clearly, this at least gives you a basis to challenge the decision and comports with the laws requirement to fully evaluate a case. Just because it is difficult in some cases does not mean they can make a blanket statement without support.
Here is a case that covers this situation:
"In order to explain the relationship between these disorders, and pursuant to its statutory duty, under 38 U.S.C. § 5107(a) (formerly § 3007), to assist claimants who have presented well-grounded claims, the Board should have ordered (as it will be required to do upon remand unless it determines, on the basis of the current evidentiary record, that the veteran is entitled to a 100% rating), under 38 C.F.R. § 3.327 (1991), a thorough, fully informed, and comprehensive medical examination to (1) reconcile the diagnoses; (2) evaluate the veteran for the purpose of determining the existence of each of the impairments noted in the record; (3) determine the degree of disability, in terms of ordinary conditions of life, imposed by each impairment, if possible, and its relation to service; and (4) determine the extent to which each impairment contributes to the veteran's employability or unemployability. See Proscelle v. Derwinski, 2 Vet.App. , , No. 90-570, slip op. at 4, 6 (U.S. Vet. App. July 24, 1992); Cousino v. Derwinski, 1 Vet.App. 536, 540 (1991); EF v. Derwinski, 1 Vet.App. 324, 326 (1991); Green v. Derwinski, 1 Vet.App. 121, 124 (1991). The examiner must have the full medical record of the veteran prior to making the evaluation. See 38 C.F.R. § 4.1. Only after making such findings may the Board properly determine the appropriate disability rating for the veteran."
Shoemaker v. Derwinski, 3 Vet. App. 248, 254-255 (Vet. App. 1992)
All that said, the errors described above are very common in cases like these. The key to successful rating is a full evaluation at the MEB level.
