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I need to create the circuit equivalent of a SPDT switch.  I thought it would be simple, with a couple transistors, but I'm out of my element.  I want it to be able to pass 1 amp at 3-5 volts.  A low state on an Arduino pin would cause it to be in one position, and a high pin would switch to the other position.  I don't need a middle "off" position.  

I drew a schematic, but can't figure out how to upload it here.  (I know, pretty pathetic).
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2 Digital pins, 2 transistors, 2 limiting resistors.  Code so one or other of the pins is high at once. Or use a double throw relay and one pin. Or at least thats the way I'd do it.  
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Or, to overcomplicate things, a servo and a SPDT switch.
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Above is a picture of what I wanted to do.  I want to create an Arduino-monitored power supply.  I have two batteries and I need to be able to switch between them quickly.  I can put a capacitor across the output to handle the changeover, but I need it to be an uninterrupted power supply.  So a relay is probably too slow.

The other problem is that I want one battery to work without any power signal (digital pin low).  That way, it gets power without any input by the Arduino, and can power itself up.

I want to turn this into a shield (my first attempt) with a bunch of other stuff on it to monitor the power consumption etc.  That part is straightforward, this switchover is messing me up.  I'm trying to configure an NPN and PNP transistor pair.  The PNP would be normally closed with a low base voltage, so it's automatically on.  Make sense?

Is there a better, smarter way to do this?
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Solid-state relays?

I don't know much about them.  Might not make sense, just a suggestion.  
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If all you want to do is have a UPS from a pair of power supplies then couldn't you simply use a diode in each of the supply legs (In effect an OR circuit).  If one supply fails, its diode will become reverse biased and effectively isolate that supply from the load.  If you want to be able to monitor which supply is providing power (and it could be both via load sharing), simply monitor the voltage upstream of the diodes; the higher unit will be the supply source.
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