@sevenstringer37, there isn't a "sensor" disconnecting batteries to worry about. If you want to change batteries less often, then two 9 volts in parallel are what you want, with (possibly) no additional power switching required (just solder both -9V wires to the switch terminal of the jack, and make sure both batteries are sufficiently insulated/there's no short allowing either battery (-) wire an alternative path to ground while there's no cable plug inserted into the jack).
More in-depth explanation below, for the sake of anyone unacquainted:
In standard Fly wiring, the separate magnetic and piezo signals each pass through the preamp
in series, and their buffered signals are outputted separately to two of the four terminals on the switching TRS jack: The buffered magnetic signal is outputted to the jack's "tip" terminal, and the piezo signal is outputted to the "ring" terminal. This allows for a stereo T/R/S cable plug to be used to keep the signals separate. When a mono T/S plug is inserted into the jack, the two signals converge; and are summed in mono. The preamp buffers the magnetic circuit signal specifically so that the two different signals will sound inoffensive when summed together in mono. The other two terminals on the jack include the "sleeve" terminal which acts as the ground connection, and an isolated terminal which gets connected to the sleeve terminal when a 1/4" jack plug is inserted - This is the battery switch terminal, and it works by completing the power circuit via connecting the battery (-) wire to ground when the inserted jack plug bridges the sleeve and battery switch terminals of the jack. The completion of this circuit allows the preamp to begin drawing current from the battery.
This is all for the sake of saying that the passive pickup signal is
not outputted when the preamp is unpowered - That requires
a preamp bypass mod utilizing a mechanical switch because; unfortunately, the Powerchip preamp doesn't contain an input bypass relay to disengage when powered on. Rest assured, nothing to do with the two active circuits being used together will defeat the "smart switching" of the Powerchip, provided all signal-carrying wires are utilized in the same way as with a passive magnetic pickup setup.
The active pickup preamp for the EMGs work in much the same manner as the Powerchip; requiring the battery (-) wire to connect to ground.
Because the max current of a 9 volt battery sufficiently exceeds the combined current draw demands of both preamps, you can power the two preamps off a single battery by connecting the battery (+) wire in parallel to each preamp's power input (with power-on switching still performed via the single battery (-) wire at the jack). However - as you're already aware -, the additional current draw will result in the battery charge depleting faster (though not necessarily
twice as fast).
Concerning using two batteries: Though other preamp designs (e.g., the Graph Tech Acousti-Phonic) allow for 18 volt operation through having two 9 volt batteries wired in series, I couldn't turn up any info concerning the Powerchip being able to safely run on power in excess of 9 volts (we know, per page 11 of the manual, that it utilizes a voltage doubler to bring its peak voltage up to 15v from a 9v input). And, though Fishman has diagrams showing that their Fluence pickups will run on 18 volts, I see only 9 volt power supplies used in Powerchip diagrams; including the 9v universal battery pack able to be used simultaneously with the Powerchip and Fluence preamps.
Even if using two batteries in series ends up being an option, the rate of power consumption would remain the same; with both batteries being drained just as quickly as in a single battery, half-power setup. So the solution (where battery life is concerned) is in utilizing parallel, isolated power supplies for each preamp; with each battery's (-) wire soldered to the switch terminal of the 4 terminal TRS jack. So long as sufficient insulation prevents either battery clip or battery casing from providing the other battery's (-) wire an alternative path to ground, the power circuit should remain uncompleted until a cable plug is inserted into the jack; powering on both preamps. Alternatively, you can route each of the battery (-) wires through a two pole switch to act as a cleaner on/off switch to avoid draining power if you leave the cable plugged in.