G. Beale (NIGTC Rev, p741) suggests that the dative here might be a dative of agency with the passive ἠγοράσθησαν. I don't see this as a possibility. If τῷ ἀρνίῳ was relocated and not connected with καὶ to τῷ θεῷ then perhaps it would be a possible reading but given the text as we have it here it doesn't seem possible. Beale's merely suggests (doesn't promote the reading) that τῷ θεῷ καὶ τῷ ἀρνίῳ might be a compound agent. But this doesn't look right with τὸ ἀρνίον being the agent of ἠγόρασας in Rev 5:9.Rev. 14:4 οὗτοί εἰσιν οἳ μετὰ γυναικῶν οὐκ ἐμολύνθησαν, παρθένοι γάρ εἰσιν, οὗτοι οἱ ἀκολουθοῦντες τῷ ἀρνίῳ ὅπου ἂν ὑπάγῃ. οὗτοι ἠγοράσθησαν ἀπὸ τῶν ἀνθρώπων ἀπαρχὴ τῷ θεῷ καὶ τῷ ἀρνίῳ,
ἠγόρασας τῷ θεῷ ἐν τῷ αἵματί σου ἐκ πάσης φυλῆς καὶ γλώσσης καὶ λαοῦ καὶ ἔθνους
There are numerous other problems to resolve with Rev. 14:4, not things we talk about in this forum.