Are the two fields globally unique, or just unique for the record they are attached to?  If they are globally unique, you could combine both field values into a single string and set that as the external id, assuming you aren’t already setting a different value in the externalid.  Netsuite will prevent duplicate creation and throw a DUP_CSTM_RCRD_ENTRY error.  If the two values are not globally unique, then maybe a combination of those two values and the internal id of the record they are being attached to would work instead.  For example:  custrecord_field1 + ‘~’ + custrecord_field2   or   otherRecord.id + ‘~’ + custrecord_field1 + ‘~’ + custrecord_field2, using whatever separator character makes sense.