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Yesterday I added a 5V/2A (phone charger?) power supply to it which seems to work fine.
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But when I plug in a second 2Pac this second one becomes active and the first one seems to drop in speed and the blue/white led stops flashing.
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Your power supply is too weak. At the very best, you are going to get 10W out of that power supply, but likely much lower. One 2PAC can pull over 10W depending on your settings.