you can do this, if you have a battery that has 2/4 elements.
but this is not good because the first half, the one that powers the 3V will discharge faster. so it is not a good way.
you could use a voltage regulator and get 3V from the 6V. this way all the battery is discharged at the same time.
What I trying to do is power a PIC micro an a motion dector off the same battery pack. Also use 1 of the microcontroler's output pin for a switch to VSS.
Im trying to get away from using a "optocupler" to keep the circut simple as possable.
Be careful, if you connect circuits together with different power supplies.
I most case they need the same grounding. So you must see it as a whole
circuit. Also the voltage to the input of an integrate circuit may higher than there power supply of the IC itself..