38 CFR §4.114

Do not combine ratings under diagnostic codes 7301 through 7329 inclusive, 7331, 7342, 7345 through 7350 inclusive, 7352, and 7355 through 7357 inclusive, with each other. Instead, when more than one rating is warranted under those diagnostic codes, assign a single evaluation under the diagnostic code that reflects the predominant disability picture, and elevate it to the next higher evaluation if warranted by the severity of the overall disability.