How do we carry older twins to the fourth floor?

Hi all, we are parents of four month-old twins and could use some advice how to carry them up the stairs once they get older and heavier.

We are living in the fourth floor in a Berlin apartment complex without elevators, and we are increasingly concerned how to carry them when one of us is alone, particularly when we also just finished a grocery shopping run or similar. Right now, they are only 4 and 6 kilos, but in a few months we might not be able to carry them in one go.

Any experience or advice would be greatly appreciated!