It makes only sense to put voltage sources in parallel if they have the same voltage, otherwise there would be infinite current (ideal voltage soucre can provide infinite current).
Two voltage sorces in parallel is exactly the same as one.
Real voltage sources always have internal resitance or can simulate an ideal voltage source only up to some maximum current. So it makes sense to connect two voltage sources in parallel to increases the maximum current that can be provided.
(If both sources don't provide exactly the same voltage precausions have to be taken that the maximum current isn't exceeded just by compensation the voltage difference. E.g. by adding a small resistor in series to both voltage sources).