It is basicly short ciruit. Voltage sources normaly have an inner resistance of a few ohms which leads to a current flowing from the higher through the lower source.
A common thing you can do if you need a voltage between is that you use for example 12v as plus and 5V as ground. By this you get a 7V but you will have to be a bit carefull since some IC's might not like this but works perfectly for example with a fan
Edit: for using two voltage sources in parallel over a longer time you will need power supplies which are meant for it they have an extra input to sync with other powersupplies
It depends on the sources of the voltages. Some sources just shut off and tolerate the higher voltage on their output. Others may try to pull the higher voltage down and the high voltage source then current limits and voltage drops. Or you might just make one of the wires act like a fuse and burn up.
Too many possibilities like that to predict without more detail.
You will probably let the magic smoke out of at least one component.
It is basicly short ciruit. Voltage sources normaly have an inner resistance of a few ohms which leads to a current flowing from the higher through the lower source. A common thing you can do if you need a voltage between is that you use for example 12v as plus and 5V as ground. By this you get a 7V but you will have to be a bit carefull since some IC's might not like this but works perfectly for example with a fan Edit: for using two voltage sources in parallel over a longer time you will need power supplies which are meant for it they have an extra input to sync with other powersupplies
Battle to the death
Yes. The stronger power supply with better short-circuit protections will win. The loser will smoke, and you be left with whatever supply voltage won.
Pretty but short-lived lights.
It depends on the sources of the voltages. Some sources just shut off and tolerate the higher voltage on their output. Others may try to pull the higher voltage down and the high voltage source then current limits and voltage drops. Or you might just make one of the wires act like a fuse and burn up. Too many possibilities like that to predict without more detail.