Diodes in parallel or series
Putting diodes in series will add the diode drops together. Reverse leakage (and capacitance) should reduce in this configuration. 

In parallel, the drop will stay the same (reverse leakage and capacitance will add), but the current capability may not be much higher, due to the possibility of thermal runaway (since as a diode gets hotter it's Vf drops, then it draws more current relative to the rest, gets hotter still, and so on). You can avoid this somewhat by placing the diodes in thermal contact with each other, and/or using a small resistor in series with each.