You will probably need to inject power at some point. It’s a simple technique, but not always easy to accomplish. This video shows how to inject power and when you will need to do it.
If you need to use more than one power supply, connect the negatives together and break the positive. This is the safest way to do it. I have heard that a load (x number of pixels) between the two supplies is enough to keep them from burning each other out, but since I have no knowledge of your electronics knowledge and for legal reasons, I’m saying break the positives.