I'll say again.......DO not series connect the devices....it will not give you the results you expect.

The only way this could ever work was if the input impedance of the two devices in series was EXACTLY the same. However it will NOT be the same and the voltage split will be different from what you expect. In addition as the current draw of one device wants to change due to different functioning state versus the other device the voltage split will change again.


One way to make such a scheme "kind of work" is to put two high power resistors (or shunt regulators) in series and connect each camera across the respective resistor. BUT the current flow through the two resistors needs to be about 10 times the maximum current load demand needed by either camera.
And even at 10X load thtough the resistors there will be a variance of the voltage split as the current draw of either camera varies. This scheme takes 11X times the normal load on the battery and the resistors will probably be huge in Watt rating unless these cameras draw just a few 10's of milliamperes.

Shunt regulators would be somewhat more effective than resistors (and note the Zener diodes somebody else proposed are a class of shunt regulator) but once again the current through the shunt path needs to be 5X the load of one camera.

In conclusion,,,,series connection of equipment like this is a BAD idea....give stuff the power supply it was designed for and you will have success.
Mike Karas