I asked a rav and he told me that two people should not say a beracha at once even if they are two men. I presume that that is because of “trei kallei la shaamei” (two voices are not heard) so they did not motzi the listeners. However, it probably does not possel the kiddushin. In general, berachot are not me’akev.
That’s if a third party needs to Motzie. Is there anyone else in this instance who needs to hear the Brocha?