Don't get confused by parallel and center-tapped. They are slightly different. I guess you are talkin pwr xfmr's, right?
If you bring the center tap to ground and use two diodes in a full wave rectification circuit, (not full wave bridge, which is four diodes), then yes, you double your current.
The catch is figuring out how the xfmr maker is specing their iron. Sometimes they cheat and advertise a current rating assuming you are going to use the standard two diode center tapped system.
This means you smoke your tranny if you run it in a full wave bridge config. and try to get their advertized juice out of it.