I personally don't ever use the 51 modifier because I have yet to see a payor not reduce the second procedure. But I found this online:
Occasionally, you can use both modifiers at the same time. For example, the FP performs a biopsy of a lesion on a patient's arm and excises a benign lesion on the neck during the same visit. You would report 11100 (Biopsy of skin, subcutaneous tissue and/or mucous membrane [including simple closure], unless otherwise listed [separate procedure]; single lesion) and 11420 (Excision, benign lesion, except skin tag [unless listed elsewhere], scalp, neck, hands, feet, genitalia; lesion diameter 0.5 cm or less) and append modifier -51 and modifier -59 to the biopsy code.
Modifier -51 is attached to the 11100 because it is the lesser-valued procedure, while -59 is attached to that code because the biopsy is a component of the excision. The -51 indicates that two procedures were performed, and the -59 indicates that they were performed on two separate lesions. “Without the -59, the payer could assume that both biopsy and excision were done on one lesion, and they wouldn't pay for that,” Newby says. The use of both codes illustrates a multiple procedure that is normally bundled but should be unbundled because it was performed on two separate sites.