You can apply multiple licenses to your FortiClient EMS to extend the license expiry. For example, consider you purchase two one-year licenses for FortiClient EMS. After you register and apply the first license, FortiClient EMS has an expiry date of August 1, 2018. You can register and apply the second license as a renewal, after which FortiClient EMS has an expiry date of August 1, 2019.
Using a second license to extend the license expiry date does not increase the number of licensed clients. To increase the number of licensed clients, contact Fortinet Support for a co-term contract.
For details, see the FortiClient EMS Administration Guide.