In this case, your payer could be bundling the two codes because either it believes a salpingoneostomy is included in the ectopic code (59120) or it views the ectopic code as a bilateral procedure. In this case, you are not billing as a bilateral procedure, but instead are billing two different procedures, one on each side. The ectopic code includes removing the fallopian tube, but 58770 represents drilling or creating a hole in the fallopian tube rather than removing it. Therefore, it would not be an integral part of 59120, and you should be able to bill for it separately.
Many coding experts prefer to use modifier -59 rather than -RT and -LT to designate that the ob-gyn performed a different procedure on the opposite side, but if your payer has indicated that you may only use -RT and -LT, that is what you should do. Regarding whether 59120 is a bilateral procedure, most people would assume that the ectopic pregnancy only occurred in one tube, not both, so the code would be unilateral. If both tubes had an ectopic pregnancy, you should be able to use modifier -50 to indicate the procedure's bilateral nature. Although CPT is silent about this, Medicare is not, indicating that 59120 is a bilateral code. Consequently, you would report this code without a modifier if the ob-gyn removed both tubes due to an ectopic pregnancy. Your payer may have adopted this opinion as well. You should appeal the denial, pointing out that the physician performed two different procedures, being sure to include information as to why the salpingo-neostomy was medically indicated.