The voltage drop should be one diodes drop + the other ones in series, so yes, if they are 0.52v each, the total is 1.04v. However, the current rating does not doube in series, it doubles in parallel. However, if the voltage drop is even slightly different between the diodes, one diode will conduct all the current and will probably burn out. The options to fix this are using low value resistors on each one to share current, or to use a larger 6A rated diode (which is better).