Stack Switch Problem in Bathroom
What you did was rewired it so that the switch going ON causes a direct short between the HOT black wire and the NEUTRAL white wire. Not a good thing at all.

As you described the original wiring with the two white wires bonded together in the back of the box was the correct connections and should have been left that way.

The old single pole switch connected the black HOT wire over to the LOAD side black wire to supply power to the light and fan. 

As described it appears that there are not sufficient wires available in the box in order to separately switch the light and the fan. To be able to achieve what you desire it will be necessary to find the downwind point that the LOAD black wire is joined to both the fan and the light and separate them at that point. Then a new wire needs to be run from that point back to your switch box so that you can independently switch the HOT black wire to the two separate LOAD black wires. 

If done correctly the place where the light and fan load wires get separated would be done within a junction box, either at the fan or at the light (depends on how it was originally installed). Then an additional Romex cable with its black/white/gnd would go back to the switch box. Now in the switch box there would be three whites bonded together in the back of the box. The three bare GND wires would be bonded together. Finally the HOT black wire would connect to both of the black screws of the stack switch and the each of the two LOAD black wires would attach to one each of the brass screws on the switch unit.