Adding a second team to work on the same product will introduce some challenges and overheads that can affect the velocity of the original team. For example, the teams will need to coordinate and communicate more frequently, integrate their work more often,resolve any dependencies or conflicts, and align their standards and practices. These activities will consume some time and effort that could otherwise be spent on delivering value. Therefore, the velocity of the original team is likely to decrease and be lower than 18, at least in the short term.

References:

Professional Scrum Master II Course, page 16: “Adding more people to a complex product development effort does not necessarily increase productivity or reduce time to market.In fact, it often has the opposite effect.”

Nexus Guide 2020, page 5: “When multiple Scrum Teams are working together on the same product, there are additional complexities that arise. These complexities can be reduced by having the teams adhere to a common definition of “Done” and a single Product Backlog.”