If your other second stage isn't freeflowing, and it's easier to breathe from it, there's no reason not to switch. Once you have switched, you can fiddle with the original second stage, while you are ascending, and see if you can get the freeflow to stop (eg. if it's due to debris in the regulator, you may be able to dislodge it and get the reg to shut off).
Even better than switching, though, is to go on your buddy's alternate. Then you are breathing a regulator which is not experiencing the malfunction yours is, and your buddy's tank should have enough reserve to get both you and him to the surface, in a stressed condition, without problems. Then, if you wish, you can shut DOWN your tank, get rid of the bubbles, the noise, and the visual interference, and surface calmly.