You could do that. You would just be connecting the two bottom halves of the bridge rectifiers in parallel.
Connecting diodes in parallel isn't a good idea unless they are closely matched and held at the same temperature. That's because one of them will have a slightly lower forward voltage than the other, and it will take more of the current, which will make it get warmer, and its forward voltage will drop, and so on, until it's carrying most or all of the current.
I don't know why there are two separate bridge rectifiers on the same transformer secondary, driving separate DC circuits. I would use a bridge rectifier rated at twice the current and supply both circuits from it, unless there's a genuine reason why you can't.