Jewett argues that the rendering "bear witness to" for Rom 8:16 "is inadmissible because this verb [συμμαρτυρέω] is in fact typically used to depict co-witnessing of some sort" (Romans, p. 500).
The text reads: αὐτὸ τὸ πνεῦμα συμμαρτυρεῖ τῷ πνεύματι ἡμῶν ὅτι ἐσμὲν τέκνα θεοῦ (UBS5).
Daniel B. Wallace argues somewhat extensively that συμμαρτυρεῖ τῷ πνεύματι is a dative of indirect object ("bear witness to") rather than a dative of association ("bear witness with"). He appeals (INTER ALIA) to BDAG in support for his argument. On the other hand, Robert Jewett (Commentary on Romans in the Hermeneia series) robustly contends that the construction in Romans 8:16 should be understood as indicating some type of co-witnessing in view of the prefix σύν and based on examples that we find in Plato's Hipp. maj. 282b1 (συμμαρτυρῆσαι δέ σοι), Plutarch (Adul. amic. 64c13) and Josephus (Ant. 19.154). Moulton-Milligan also suggests that there are texts from the papyri that favor the understanding "bear witness with."
Rogers and Rogers (The New Linguistic and Exegetical Key to the Greek New Testament, page 330): "συμμαρτυρεῖ pres. ind. act. συμμαρτυρέω (#5210) to bear witness w. someone, to confirm, to testify in support of someone. Used in the papyri where the signature of each attesting witness is accompanied by the words, 'I bear witness w. and I seal w.'" (MM).