Can you combine the outputs of two ports on a USB Battery Pack?
The specifics depend on the battery pack in question, but many usb power banks and usb supplies with multiple ports have their USB VCC output electronically tied together. The 2.4 limit is not a hard limit enforced by the power bank, rather it is a soft limit. The port is wired with the right resistor voltage divider on the usb data pairs, which tell the phone plugged into it that it's okay to draw up to that amount. This is generally the Apple standard. The phone then limits it's own draw.

That means you don't really need to use two usb ports. Drawing from the two ports help distribute the power through more copper which is nice though.

Note this applies to dumber power banks. Newer packs with Quick Charge or other features have more complicated circuitry and the same may not apply. See Ali's comment below.