You bill 2 (or more) CPT codes when the work performed is not better described by 1 code. For example, if there was a code for C-section with removal of fibroids, that is what you would use. Since there is not, you code both. Another example - laparoscopic hysterectomy with removal of tubes and ovaries. You would not code separately for laparoscopic hysterectomy and then laparoscopic tubes/ovaries since there is a code that describes both. Code:
58571 Laparoscopy, surgical, with total hysterectomy, for uterus 250 g or less; with removal of tube(s) and/or ovary(s)
NOT 58570 Laparoscopy, surgical, with total hysterectomy, for uterus 250 g or less and 58661 Laparoscopy, surgical; with removal of adnexal structures (partial or total oophorectomy and/or salpingectomy). That would be unbundling.
When you bill multiple procedures, insurance will already automatically discount the 2nd procedure by 50%. You would NOT get paid for 31 RVUs in this example.
You would get paid 100% for the highest 59514 =16
50% for second 58146 = 7.5
Modifier -51 is to indicate it was a second/third/fourth procedure. In my experience, the insurance will automatically add on the -51 when processing.
I know my local MAC had advised many years ago for us NOT to add the -51, that the carrier would add it on their end. While you adding the -51 isn't wrong, there are situations where you may short yourself some payment. In the same example, if your fee schedule is not based on RVUs and the fee for 58146 is higher than 59514, you would not get the correct payment if you added -51 to 58146.
Here's a good AAPC article about the multiple procedure payment reduction:
https://www.aapc.com/blog/41773-mppr-facts/