Nazir 2:5

If one person says, “I am a nazir and I commit to shave a nazir” (i.e., provide a nazir’s sacrifices) and his friend hears and says, “I also commit to shave a nazir,” if they are clever, they will provide sacrifices for one another. Otherwise, they will have to provide sacrifices for other nazirs.

Nazir 2:6

If a person says, “I commit to shave half a nazir” and his friend hears and says, “I also commit to shave half a nazir,” Rabbi Meir such each must provide a nazir with the entirety of his sacrifices. The Sages say that each may provide a nazir with only half of his sacrifices.