> 2 9V wired in parallel would do it and is preferable to 4 AA

Why is it preferable ? You get more bang for your buck from
4xAAs than you do from a 9V. The only place I ever use a 9V
(rechargeable) is in my DMM, because that's all that will fit.
Anything else has either a 2, 4, or 6-pack of AAs

> Is this a reasonable thing to do?  Is it dangerous at all if both are
> fresh of the same kind?  How about if not?

I'm having a guess at this, but I'd have thought the higher of two
unequal batteries would be wasting power driving the internal
resistance of the other. If the lesser of the two is flat it's dead
weight basically and just adds to the load of the survivor. And
I'm not too sure about the chemical effects of trying to recharge
one battery with another. Like June Allyson, you could get some
serious leakage problems

> Would a 5ohm resistor in series with each before being wired
> in parallel alleviate any dangers

There shouldn't be any dangers with small batteries, but any series
component wastes power as heat. Even the voltage drop of a diode.
If you can make it run on 4xAA you'll not be wasting power dropping
that higher voltage of the 9V

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