Adding a Sub Interface (VLAD ID) to Participants for Performance SLA

Hello, we have two HA Fortigates. The customer has the Internet interface split in two sub interfaces. Two different Internet connectivity with two different VLAN IDs. The interface is an aggregate one. The problem is we want to use the performance SLA over a ping to google DNS 8.8.8.8. When I select interface members I only have the option of the MAIN interface, NOT the sub interfaces. Can I use the sub interface as Participant members for the SLA Perf?