The reply from gfleming is one way to add redundancy to an 802.3ad FortiLink for certain topos. However, if for example you need two distribution switches to directly connect via FortiLink and both be active, it might be better to use the Hardware Switch FortiLink.
To do this, you'll need to first get rid of your 802.3ad FortiLink interface - which means getting rid of all references to it. Most of this will need to be done in the CLI. Before you start, it would be helpful to delete any switches that have already been authorized. Then you'll want to conf sys interface, edit <FortiLink Interface>, and then disable FortiLink via set fortilink disable. Next, you'll want to delete the DHCP and DHCP6 servers on the interface via conf sys dhcp server and conf sys dhcp6 server.
Hopefully that will be enough to let you then delete the 802.3ad FortiLink interface, but if not... just keep working to remove config references. Once you delete the 802.3ad FortiLink interface, you'll then want to use the GUI to create a new Hardware Switch. Go to Network > Interfaces and click the Create button to create a new interface. Select the Hardware Switch type and select the member interfaces that will be part of it. Save it, then go back to the CLI and enable FortiLink on that new interface via conf sys int, edit <New FortiLink Interface>, and set fortilink enable. At this point, you should be ready to start authorizing your switches again.
I hope this is helpful!