Switch between the current source with the highest amperage
Amperage does not work the way that you are describing.

Basically, a power supply puts out a specific voltage, and then the device draws whatever amperage it is designed to draw (assuming the power supply can put out enough voltage). If a device takes 1 amp at 5 volts, you can use any 5 volt power supply as long as it supplies at least 1 amp of current.

My guess is that the 1 amp power supply is not powerful enough for the tablet to detect - it's likely that the voltage is sagging down below the voltage that the tablet needs. If that is true, then you can't charge the tablet with the 1 amp power supply. 

You cannot hook two power supplies in parallel and get the sum of their outputs. 

I would look to power the tablet directly from the battery with a power supply that puts out enough to drive it directly, and then come up with another way of detecting whether the car is running. 

I would look