If you are absolutely certain the two secondary voltages are identical, and you take care to wire them in phase, you can parallel them and use a single large rectifier. If there is a risk the voltages are different, even by a small amount, you will feed current from one winding into the other and produce a lot of heat, in that case it is safer to rectify them individually and parallel the DC outputs. The result should be the same whichever way you do it.
Brian.