I have double stack switch with intention of 2 independent light areas ( undermount cabinet lighting and overhead pot light. When I connect black hot wire to black screw ( both feeds simultaneously connected via pre existing setup with double stacked switch) and black wiring independently connected out to pot light and undermount lighting through separate stacks via brass screws, pot light switches off and on independently- undermount cabinet lighting only switches on when pot light is on. Any suggestions to correct this situation would be greatly appreciated.
Electrical – How to wire a double stack switch
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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.
I believe that you have mixed up the light-hot and the panel-hot wires. So when you flip the first switch it supplies power to the (whole) switch. You need to move the light wire (yellow in the picture below) and put it on the brass screw corresponding to the one you thought was the light wire. The wire that you thought went to the light is the power/panel hot.
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Best Answer
Not knowing which brand you are using, I just googled double stacked switches, saw the Leviton 5224-2W and noticed the black screws are for the switch legs going to each light while the brass screws are for the hot common, which are typically bridged together with a removable fin (or "tab" as Brad Gilbert mentioned). If the fin has been removed, you will have to create a bridge with wire to feed both switches. When connecting, use the brass screw(s) for the constant hot and the black screws for the independent switch legs. If the connections are mis-arranged on the switch you would have the issue as you describe.
In pictorial form, if you have this:
You'll need to do this instead:
The Black and White coming in from the bottom would represent your hot/neutral feed at the switch. This is assuming some things, as your feed could just as well come in at one of the lights. A few pictures from your end would help if this does not look like what you have.