From: Rabbi Moshe Sofer
To: Rabbi Yaakov Shag
When a father is absent and appoints an agent to perform the pidyon haben, the agent should recite both the birkas hamitzvah and Shehecheyanu on his own account. The concern that two people should not recite Shehecheyanu on one mitzvah is resolved by the Rashba's ruling that this applies only when both principal and agent are present; in the father's absence no such issue arises, and the agent recites both blessings. As an added precaution the Chasam Sofer recommends placing a new fruit before the agent, though he maintains the ruling holds even without it.
Source: Shu"t Chasam Sofer YD §298