Combine 2 powerbanks for mobile

Hello,
I have 2 old powerbanks that didn't have enough cappacity. So I want to combine the 2.
But both have 5v 800ma output. So if I am afraid that the voltage or the ampere will be to high.
How can I combine the 2 without damaging my phone?
Sorry for my bad english.

Greeting,
Thijs

By using a Y- cable with one female and two male connectors.
Check the power banks for protection diodes or simply add one to each male connector on the Y-cable to be on the safe side.
Depending on the electronics you can also add the batteries from one pack to the other pack to increase capacity.