§4.114 Schedule of ratings—digestive system.
Ratings under diagnostic codes 7301 to 7329, inclusive, 7331, 7342, and 7345 to 7348 inclusive will not be combined with each other. A single evaluation will be assigned under the diagnostic code which reflects the predominant disability picture, with elevation to the next higher evaluation where the severity of the overall disability warrants such elevation.
Unfortunately, this statute would rebut your CUE, to seperate the combined IBS and GERD; but there's some good news.
My understanding of the bolded part, is that if a single rating is going to be assigned; the highest rating must be given from either 7319 and 7346 depending on the severity of the symptoms.
What does this mean? The highest rating of the two diagnostic codes would fall under 7346 Hiatal Hernia [GERD is assigned this code]at 60%. (That is the highest that is allowable.)
7346 Hernia hiatal: (hiatal hernia)
Here is a case where the veteran was granted 60% for combined GERD and IBS. Please read carefully. This case outlines exactly how the board used the medical evidence and the LAW to get it granted. You may have a CUE for getting assigned the highest rating.
https://www.va.gov/vetapp09/files2/0919318.txt
I apologize if this isn't what would be ideal to hear, but I believe I found a way for attaining a higher evaluation, under your circumstances.