KiCad Multiple power inputs
Multiple power symbols are not there to increase the maximum current. They are used when different parts of your circuit operate on different voltages. For an example you might have a digital circuit that runs at 5V DC, which controls a 12V relay. The  +5V and +12V symbols are used to distinguish between these different rails. 
If you want to increase the maximum current that can be carried in a line, you have to do it in pcbnew. When you route the board, make sure to create a net class for GND and your positive rails, and give them an higher value for track width.