Nope. USB "power banks" contain a cheap SMPS to convert the battery voltage (nominally 3.7 to 4.2 volts) to a steady 5 volts. Hardly a clean source. Standard USB ports have a nominal 0.5A current limit. (Modern USB ports can negotiate a higher current with compatible devices.) The Y cable is a kludge which allows drawing up to 1A by plugging the ends into two USB sockets on the computer. Do NOT use a Y cable to connect both a computer and a battery pack to a DAC. Consider what happens if you turn off the computer without unplugging the cable. You are likely to damage the power pack, the computer or both. If you still want to try using a battery pack, use it to power a powered hub. Powered hubs perform isolation, when you plug in the external power they disconnect the 5V feed from the computer.