Rav Shmuel Mohilever said there were two of them because there were two tasks, to take the Israelites out of Egypt and to take Egypt out of the Israelites.
Which leader was which?
You can argue either way, but perhaps the best argument is that Moses’ task was the physical one of organising the departure from Egypt and leading the people from the house of bondage onto the road to independent nationhood, whilst Aaron was the one who eradicated the traces of Egypt from the Israelite heart and mind.
Aaron as high priest busied himself with the spiritual and cultural dimensions. The worship gatherings over which he presided brought the beginnings of a national ethos. The advice he gave to Israelites in crisis situations helped them to develop an Israelite mentality.
Neither leader finished the task; each died before his work was finished. But taken together, what they created was the ability to put Egypt behind them and think of an Israelite future.