Powering 20 m LED strips
You don't need two power supplies. One is enough. Just connect the positive wire to one side of the strip and the negative to the other. Then you will have equal voltage at each LED group and this will ensure equal brightness of all LEDs.







simulate this circuit &ndash; Schematic created using CircuitLab

Using two separate power supplies has some drawbacks:


In the middle of the strip voltage will be lower than both ends. LEDs in the middle will shine less than LEDs at both ends.
One of the supplies may fail and not be noticed for long time, until someone sees the difference from one end to the other.
Two power supplies almost always are more expensive and installing mains supply (230VAC/115VAC) connection leads to additional costs.


Update:
When current through the strip is higher that the strip could handle safe, you need to cut the strip to pieces and power each piece separately. However the cut pieces can also be powered as I suggest above to ensure equal voltages and brightness.