Why did Moses strike the rock twice and what was Aaron's participation?
"So Moses and Aaron went from the presence of the assembly to the door of the tabernacle of meeting, and they fell on their faces. And the glory of the LORD appeared to them. Then the LORD spoke to Moses, saying, "Take the rod; you and your brother Aaron gather the congregation together. Speak to the rock before their eyes, and it will yield its water; thus you shall bring water for them out of the rock, and give drink to the congregation and their animals." So Moses took the rod from before the LORD as He commanded him. And Moses and Aaron gathered the assembly together before the rock; and he said to them, "Hear now, you rebels! Must we bring water for you out of this rock?" Then Moses lifted his hand and struck the rock twice with his rod; and water came out abundantly, and the congregation and their animals drank. Then the LORD spoke to Moses and Aaron, "Because you did not believe Me, to hallow Me in the eyes of the children of Israel, therefore you shall not bring this assembly into the land which I have given them" (Numbers 20:6-12).
Beyond a simple statement of fact, striking the rock twice tells us that this wasn't an accident. Why Moses struck the rock can be explained by two things. First, this is the second time God supplied water to Israel. The first time Moses was told to strike the rock (Exodus 17:6). Second, God told him specifically to bring Aaron's rod. We can understand that Moses assumed that the rod was requested because it was to be used.
Aaron's involvement was being there, thus, showing approval to what was being done, and it was Aaron's rod which was being used by Moses. God considers those who give support to wrong as equally guilty as those who do wrong (Romans 1:32).
Moses and Aaron's failure was two-fold. In scolding the people they asked if they (Moses and Aaron) had to bring forth water. They did not give God the credit as they should have. Secondly, they did not do as God commanded. They improvised, but in doing so they introduced doubt into this miracle. One could claim that the water was there and Moses merely knocked a stone covering the spring aside. If Moses had done it God's way it would have been more impressive because merely speaking to a rock would not allow water to flow.
Thanks for your insight and may God bless you!