Answer: You have three options to code the NSTs. You can bill 59025-26 (Fetal non-stress test; professional component) x 2. Or you can submit 59025-26 and 59025-26-51 (Multiple procedures). And finally, you could report 59025-26 and 59025-26-59 (Distinct procedural service).
CPT implies that the last option is the most correct. After the entry for 76818-76819 (Fetal biophysical profile ), it states, Fetal biophysical profile assessments for the second and any additional fetuses should be reported separately by code 76818 or 76819 with the modifier -59 appended. So your best option may be to report 59025-26 and 59025-26-59. But be sure to check with your carriers to determine their coding guidelines for reporting these services.